Protective cover sheet comprising a UV-absorbing layer for a polarizer plate and method of making the same

ABSTRACT

The invention relates to a protective cover sheet comprising a low birefringence protective polymer film and a layer that promotes adhesion to poly(vinyl alcohol), the protective cover sheet comprising at least one functional layer containing a UV-absorbing polymer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 11/106,789,filed on Apr. 15, 2005.

FIELD OF THE INVENTION

The present invention generally relates to low birefingence protectivepolymer films used in protective cover sheets for polarizer plates animproved method for producing polarizing plates, and an electronicdisplays employing the same. More particularly, the invention relates toa protective cover sheet comprising one or more functional layers,including a layer that promotes adhesion to polyvinyl alcohol-containingdichroic films, wherein a UV-absorbing polymer is contained in at leastone of the functional layers.

BACKGROUND OF THE INVENTION

Transparent resin films are used in a variety of optical applications.For example, a number of different optical elements in Liquid CrystalDisplays (“LCDs”) may be formed from resin films. The structure of LCDsmay include a liquid crystal cell, one or more polarizer plates, and oneor more light management films. Liquid crystal cells are formed byconfining liquid crystals such as vertically-aligned (VA), in-planeswitching (IPS), twisted nematic (TN) or super twisted nematic (STN)materials between two electrode substrates. Polarizer plates aretypically a multi-layer element comprising resin films. In particular, apolarizer plate can comprise a polarizing film sandwiched between twoprotective cover sheets that comprise a low birefingence protectivepolymer film.

Polarizing films are normally prepared from a transparent and highlyuniform, amorphous resin film that is subsequently stretched to orientthe polymer molecules and then stained with a dye to produce dichroicfilm. An example of a suitable resin for the formation of polarizerfilms is fully hydrolyzed poly(vinyl alcohol) (PVA). Because thestretched PVA films used to form polarizers are very fragile anddimensionally unstable, protective cover sheets are normally laminatedto both sides of the PVA film to offer both support and abrasionresistance.

Protective cover sheets used in polarizer plates are required to havehigh uniformity, good dimensional and chemical stability, and hightransparency. Originally, protective coversheets were formed from glass,but a number of resin films are now used to produce lightweight andflexible polarizers. Many resins have been suggested for use inprotective cover sheets including cellulosics, acrylics, cyclic olefinpolymers, polycarbonates, and sulfones. However, acetyl cellulosepolymers are most commonly used in protective cover sheets for polarizerplates. Polymers of the acetyl cellulose type are commercially availablein a variety of molecular weights as well as the degree of acylsubstitution of the hydroxyl groups on the cellulose backbone. Of these,the fully substituted polymer, triacetyl cellulose (TAC) is commonlyused to manufacture resin films for use in protective cover sheets forpolarizer plates.

The cover sheet normally requires a surface treatment to insure goodadhesion to the PVA dichroic film. When TAC is used as the protectivecover film of a polarizer plate, the TAC film is subjected to treatmentin an alkali bath to saponify the TAC surface to provide suitableadhesion to the PVA dichroic film. The alkali treatment uses an aqueoussolution containing a hydroxide of an alkali metal, such as sodiumhydroxide or potassium hydroxide. After alkali treatment, the celluloseacetate film is typically washed with weak acid solution followed byrinsing with water and drying. This saponification process is both messyand time consuming.

U.S. Pat. No. 2,362,580 describes a laminar structure wherein twocellulose ester films each having a surface layer containing cellulosenitrate and a modified PVA is adhered to both sides of a PVA film. JP0609491 SA discloses a protective film for polarizer plates wherein theprotective film has a hydrophilic layer which provides adhesion to PVAfilm. Commonly-assigned, copending U.S. patent application Ser. No.10/838,841, filed May 4, 2004 describes a guarded protective cover sheethaving a removable, carrier substrate and a cover sheet comprising a lowbirefringence protective polymer film and a layer promoting adhesion topoly(vinyl alcohol) on the same side of the carrier substrate as the lowbirefringence protective polymer film which eliminates the need for thesaponification process.

Protective cover sheets may be a composite or multilayer film includingother functional layers (herein also referred to as auxiliary layers)such as an antiglare layer, antireflection layer, anti-smudge layer,compensation layer, or antistatic layer. Generally, these functionallayers are applied in a process step that is separate from themanufacture of the low-birefringence protective polymer film, but may belater applied to a form a composite film. A functional or auxiliary filmmay combine functions of more than one functional layer, or a protectivepolymer film may also serve the function of a functional layer.

For example, some LCD device may contain a low birefringence protectivepolymer film that also serves as a compensation film to improve theviewing angle of an image. Compensation films (i.e. retardation films orphase difference films) are normally prepared from amorphous films thathave a controlled level of birefringence prepared, for example, eitherby uniaxial stretching or by coating with discotic dyes. Suitable resinssuggested for formation of compensation films by stretching includepoly(vinyl alcohol)s, polycarbonates and sulfones. Compensation filmsprepared by treatment with dyes normally require highly transparentfilms having low birefringence such as TAC and cyclic olefin polymers.

Examples of optical films prepared by casting methods include: (1)Cellulose acetate sheets used to prepare light polarizing films asdisclosed in U.S. Pat. No. 4,895,769 to Land and U.S. Pat. No. 5,925,289to Cael as well as more recent disclosures in U.S. Patent Application.2001/0039319 A1 to Harita and U.S. Patent Application 2002/001700 A1 toSanefiji; (2) Cellulose triacetate sheets used for protective covers forlight polarizing films as disclosed in U.S. Pat. No. 5,695,694 to Iwata;(3) Polycarbonate sheets used for protective covers for light polarizingfilms or for retardation plates as disclosed in U.S. Pat. No. 5,818,559to Yoshida and U.S. Pat. Nos. 5,478,518 and 5,561,180 both to Taketani;and (4) Polyethersulfone sheets used for protective covers for lightpolarizing films or for retardation plates as disclosed in U.S. Pat.Nos. 5,759,449 and 5,958,305 both to Shiro.

It would be desirable to further improve the optical properties of afilm and protective cover sheets to increase the amount of light outputin a display. The use of UV dyes in the prior art have caused problemsof migrating to the surface causing surface haze and light scatteringwhich reduces optical transmittance of the cover sheet.

It would be also very advantageous to avoid the need for saponificationof protective cover sheets in the preparation of polarizer plates fromresin films which requires a lamination process involving pretreatmentin an alkali bath and then application of adhesives, pressure, and hightemperatures. Avoiding such a saponification operation would improveboth productivity and reduce the necessary conveyance and handling ofthe sheets. Although advantageous for protective cover sheets ingeneral, this would be especially desirable for relatively thinnerprotective cover sheets.

SUMMARY OF THE INVENTION

It is an object of the invention eliminate or reduce migrating UV dyefrom a low birefringence protective polymer film.

It is a further object to provides a low birefringence protectivepolymer film having a film surface appearance and haze level that isless sensitive to coating/drying process conditions.

It is an object of the present invention to overcome the limitations ofprior-art polarizer cover sheets and to provide an improved cover sheetthat eliminates the need for complex surface treatments such assaponification prior to the fabrication of polarizer plates.

It is a still further object to provide an improved process for thefabrication of polarizer plates using the novel cover sheets of theinvention.

These and other objects of the invention are accomplished by an improvedadhesion-promoting layer for adhering polyvinyl alcohol to lowbirefringence protective polymer films and the presence of aUV-absorbing layer in the same or another function layer.

In particular, the invention is directed to a multilayer protectivecover sheet comprising a low birefringence protective polymer film andone or more functional layers, which layers are either adjacent ornon-adjacent each other, wherein the one or more functional layerscomprise at least an adhesion-promoting layer for adhering a poly(vinylalcohol)-containing film to said low birefringence protective polymerfilm, wherein at least one of said one or more functional layerscomprise a UV-absorbing polymer.

The invention is also directed to a guarded cover sheet compositecomprising a carrier substrate and such a multilayer protective coversheet.

Finally, the invention is also directed to a method of forming apolarizing plate comprising:

(A) providing two guarded cover sheet composites each comprising:

-   -   (i) a carrier substrate; and    -   (ii) a multilayer protective cover sheet according to the        invention as described herein;

(B) providing a poly(vinyl alcohol)-containing dichroic film; and

(C) simultaneously or sequentially bringing each protective cover sheetinto contact with said poly(vinyl alcohol)-containing dichroic film suchthat the layer promoting adhesion to a poly(vinyl alcohol)-containingfilm in each protective cover sheet is in contact with said poly(vinylalcohol)-containing dichroic film.

Protective cover sheets in accordance with the present inventionadvantageously eliminate or reduce migration of UV dye, which mightotherwise adversely affect subsequent coating processes and filmproperties. The protective cover sheets are characterized by filmsurface appearance and haze level that exhibits less sensitivity tocoating or drying process conditions.

Protective cover sheets of the invention comprising an adhesionabsorbing layer provide excellent adhesion to polyvinylalcohol-containing dichroic films and eliminate the need to alkali treatthe cover sheets prior to lamination to the dichroic films, therebysimplifying the process to manufacture polarizing plates.

Optionally, the multilayer protective cover sheet can comprise variouscombinations of functional or auxiliary layers, for example, anabrasion-resistant layer, antiglare layer, low reflection layer,antireflection layer, antistatic layer, viewing angle compensationlayer, and/or moisture barrier layer, or combinations thereof, may beemployed in the cover sheets of the invention.

In one embodiment, the invention is especially advantageous for themanufacture of relatively very thin cover sheets of the invention, whichis facilitated by applying the cover sheet coating formulation onto adiscontinuous carrier substrate that supports the wet cover sheet filmthrough the drying process and eliminates the need to peel the sheetfrom a metal band or drum prior to a final drying step as typicallyperformed in the casting methods described in prior art. Rather, thecover sheet is substantially completely dried before separation from thecarrier substrate. In fact, the composite comprising the cover sheet andcarrier substrate are preferably wound into rolls and stored untilneeded for the fabrication of polarizer plates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an exemplary coating and drying apparatus thatcan be used in the practice of the method of the present invention;

FIG. 2 is a schematic of an exemplary coating and drying apparatus as inFIG. 1 but also including a station where an alternate winding operationfurther comprises application of a strippable protection layer;

FIG. 3 is a schematic of an exemplary multi-slot coating apparatus thatcan be used in the practice of the present invention;

FIG. 4 is a schematic of an exemplary casting apparatus that can be usedin the practice of the present invention;

FIG. 5 shows a cross-sectional representation of a three-layer coversheet of the invention;

FIG. 6 shows a cross-sectional representation of a guarded cover sheetof the invention comprising a three-layer cover sheet and a partiallypeeled carrier substrate;

FIG. 7 shows a cross-sectional representation of a guarded cover sheetof the invention comprising a four-layer cover sheet and a partiallypeeled carrier substrate;

FIG. 8 shows a cross-sectional representation of a guarded cover sheetof the invention comprising a four-layer cover sheet and a partiallypeeled carrier substrate wherein the carrier substrate has a releaselayer formed thereon;

FIG. 9 shows a schematic of a method to fabricate a polarizer plateusing the guarded cover sheet composites of the invention; and

FIG. 10 shows a cross-sectional representation of a liquid crystal cellwith polarizer plates on either side of the cell in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions apply to the description herein:

In-plane phase retardation, R_(in), of a layer is a quantity defined by(nx−ny)d, where nx and ny are indices of refraction in the direction ofx and y; x is taken as the direction of maximum index of refraction inthe x-y plane and y direction is taken perpendicular to it; the x-yplane is parallel to the surface plane of the layer; and d is athickness of the layer in the z-direction. The quantity (nx−ny) isreferred to as in-plane birefringence, Δn_(in). The value of Δn_(in) isgiven at a wavelength λ=550 nm n.

Out of-plane phase retardation R_(th), of a layer is a quantity definedby [nz−(nx+ny)/2]d, where nz is the index of refraction in thez-direction. The quantity [nz−(nx+ny)/2] is referred to as out-of-planebirefringence, Δn_(th). If nz>(nx+ny)/2, Δn_(th) is positive (positivebirefringence), and thus the corresponding R_(th) is also positive. Ifnz<(nx+ny)/2, Δn_(th) is negative (negative birefringence) and R_(th) isalso negative. The value of Δn_(th) is given at λ=550 mm.

Intrinsic Birefringence, Δn_(int), of a polymer refers to the quantitydefined by (ne−no), where ne and no are the extraordinary and theordinary index of the polymer, respectively. The actual birefringence(in-plane Δn_(in) or out-of-plane Δn_(th)) of a polymer layer depends onthe process of forming it, thus the parameter Δn_(int).

Amorphous is defined as a lack of long-range order. Thus an amorphouspolymer does not show long-range order as measured by techniques such asX-ray diffraction.

Transmission is a quantity to measure the optical transmissivity. It isgiven by the percentile ratio of out coming light intensity I_(out) toinput light intensity I_(in) as I_(out)/I_(in)×100.

Optic Axis refers to the direction in which propagating light does notsee birefringence.

Uniaxial is defined as two of the three indices of refraction, nx, ny,and nz, are essentially the same.

Biaxial is defined as the three indices of refraction, nx, ny, and nz,are all different.

Cover sheets employed in Liquid Crystal Displays are typically polymericsheets having low optical birefringence that are employed on each sideof a dichroic PVA film in order to maintain the dimensional stability ofthe dichroic film and to protect it from moisture and UV degradation. Inthe following description, a guarded cover sheet means a cover sheetthat is disposed on a removable, protective carrier substrate. Astrippable, protective film may also be employed on the side of thecover sheet opposite to the carrier substrate so that both sides of thecover sheet are protected prior to its use in a polarizer plate.

A layer promoting adhesion to PVA is a distinct layer that is applied ina coating step either separate from or simultaneous with the applicationof the low birefringence polymer film. The layer promoting adhesion toPVA provides acceptable adhesion of the cover sheet to a PVA dichroicfilm (in a liquid crystal display application) without the need for awet pretreatment, such as saponification, of the cover sheet prior tolamination to the PVA film.

The present invention is directed to an improved adhesion-promotinglayer for adhering polyvinyl alcohol to low birefringence protectivepolymer films. The adhesion-promoting layer of the invention compriseswater-soluble polymer and hydrophobic polymer particles. In particular,the present invention provides a protective cover sheet for polarizingplates comprising a low birefringence protective polymer film and alayer containing a water-soluble polymer and polymer particles thatpromotes adhesion to polyvinyl alcohol-containing dichroic films.

As mentioned above, the cover sheet of the invention can also comprisesone or more auxiliary layers such as an abrasion resistant hardcoatlayer, antiglare layer, anti-smudge layer or stain-resistant layer,antireflection layer, low reflection layer, antistatic layer, viewingangle compensation layer, and moisture barrier layer.

The present invention also provides a guarded cover sheet compositecomprising a carrier substrate, a low birefringence polymer film, alayer containing a water-soluble polymer and polymer particles thatpromotes adhesion to polyvinyl alcohol, and optionally one or moreauxiliary layers on the same side of said carrier substrate as the lowbirefringence polymer film. Optionally, the guarded cover sheetcomposite of the invention also comprises a strippable, protection layeron the side of the cover sheet opposite to the carrier substrate. Theguarded cover sheet composite is particularly effective when the lowbirefringence protective polymer film is relatively thin, for example,when the thickness is about 40 micrometers or less, especially 15 to 30micrometers thick.

Turning now to FIG. 1 there is shown a schematic of an exemplary andwell-known coating and drying system 10 suitable for preparing the coversheets of the present invention. The coating and drying system 10 may beused to apply very thin films to a moving carrier substrate 12 and tosubsequently remove solvent in a dryer 14. A single coating apparatus 16is shown such that system 10 has only one coating application point andonly one dryer 14, but two or three (even as many as six) additionalcoating application points with corresponding drying sections are knownin the fabrication of composite thin films. The process of sequentialapplication and drying is known in the art as a tandem coatingoperation.

Coating and drying system 10 includes an unwinding station 18 to feedthe moving carrier substrate 12 around a back-up roller 20 where thecoating is applied by coating apparatus 16. The coated substrate 22 thenproceeds through the dryer 14. In one embodiment of the presentinvention, a guarded cover sheet composite 24 comprising a cover sheeton substrate 12 is wound into rolls at a wind-up station 26.

As depicted, an exemplary four-layer coating is applied to moving web12. Coating liquid for each layer is held in respective coating supplyvessel 28, 30, 32, 34. The coating liquid is delivered by pumps 36, 38,40, 42 from the coating supply vessels to the coating apparatus 16 viaconduits 44, 46, 48, 50, respectively. In addition, coating and dryingsystem 10 may also include electrical discharge devices, such as coronaor glow discharge device 52, or polar charge assist device 54, to modifythe substrate 12 prior to application of the coating.

Turning next to FIG. 2 there is shown a schematic of the same exemplarycoating and drying system 10 depicted in FIG. 1 with an alternativewinding operation to apply a strippable protection layer. Accordingly,the figures are numbered identically up to the winding operation. In thepractice of the present invention the guarded cover sheet composite 24comprising a carrier substrate (which may be a resin film, paper,resin-coated paper, or metal) with a cover sheet applied thereto istaken between opposing nip rollers 56, 58. The guarded cover sheetcomposite 24 is adhesively adhered or electrostatically adhered to apreformed strippable protection layer 60 which is supplied fromunwinding station 62 and the guarded cover sheet composite containingthe preformed strippable protection layer 60 is wound into rolls atwind-up station 64. In a preferred embodiment of the present invention,polyolefin or polyethylene phthalate (PET) is used as the preformedstrippable protection layer 60. Either the guarded cover sheet composite24 or the protection layer 60 may be pretreated with an electric chargegenerator to enhance the electrostatic attraction of the protectionlayer 60 to the guarded cover sheet composite 24.

The coating apparatus 16 used to deliver coating fluids to the movingsubstrate 12 may be a multi-layer applicator such as a slide beadhopper, as taught for example in U.S. Pat. No. 2,761,791 to Russell, ora slide curtain hopper, as taught by U.S. Pat. No. 3,508,947 to Hughes.Alternatively, the coating apparatus 16 may be a single layerapplicator, such as slot die bead hopper or jet hopper. In a preferredembodiment of the present invention, the coating apparatus 16 is amulti-layer slide bead hopper.

As shown in FIGS. 1 and 2, coating and drying system 10 includes a dryer14 that will typically be a drying oven to remove solvent from thecoated film. An exemplary dryer 14 used in the practice of the method ofthe present invention includes a first drying section 66 followed byeight additional drying sections 68-82 capable of independent control oftemperature and air flow. Although dryer 14 is shown as having nineindependent drying sections, drying ovens with fewer compartments arewell known and may be used to practice the method of the presentinvention. In a preferred embodiment of the present invention the dryer14 has at least two independent drying zones or sections.

Preferably, each of drying sections 66-82 has independent temperatureand airflow controls. In each section, temperature may be adjustedbetween 5° C. and 150° C. To minimize drying defects from case hardeningor skinning-over of the wet layers, optimum drying rates are needed inthe early sections of dryer 14. There are a number of artifacts createdwhen temperatures in the early drying zones are inappropriate. Forexample, fogging or blush of cellulose acetate films is observed whenthe temperature in zones 66, 68 and 70 are set at 25° C. This blushdefect is particularly problematic when high vapor pressures solvents(methylene chloride and acetone) are used in the coating fluids.Aggressively high temperatures of 95° C. in the early drying sections66, 68, and 70 tend to cause premature delamination of the cover sheetfrom the carrier substrate. Higher temperatures in the early dryingsections are also associated with other artifacts such as casehardening, reticulation patterns and blistering of the cover sheet.

In a preferred embodiment of the present invention, the first dryingsection 66 is operated at a temperature of at least about 25° C. butless than 95° C. with no direct air impingement on the wet coating ofthe coated substrate 22. In another preferred embodiment of the methodof the present invention, drying sections 68 and 70 are also operated ata temperature of at least about 25° C. but less than 95° C. It ispreferred that initial drying sections 66, 68 be operated attemperatures between about 30° C. and about 60° C. It is most preferredthat initial drying sections 66, 68 be operated at temperatures betweenabout 30° C. and about 50° C. The actual drying temperature in dryingsections 66, 68 may optimize empirically within these ranges by thoseskilled in the art.

Referring now to FIG. 3, a schematic of an exemplary coating apparatus16 is shown in detail. Coating apparatus 16, schematically shown in sideelevational cross-section, includes a front section 92, a second section94, a third section 96, a fourth section 98, and a back plate 100. Thereis an inlet 102 into second section 94 for supplying coating liquid tofirst metering slot 104 via pump 106 to thereby form a lowermost layer108. There is an inlet 110 into third section 96 for supplying coatingliquid to second metering slot 112 via pump 114 to form layer 116. Thereis an inlet 118 into fourth section 98 for supplying coating liquid tometering slot 120 via pump 122 to form layer 124. There is an inlet 126into back plate 100 for supplying coating liquid to metering slot 128via pump 130 to form layer 132. Each slot 104, 112, 120, 128 includes atransverse distribution cavity. Front section 92 includes an inclinedslide surface 134, and a coating lip 136. There is a second inclinedslide surface 138 at the top of second section 94. There is a thirdinclined slide surface 140 at the top of third section 96. There is afourth inclined slide surface 142 at the top of fourth section 98. Backplate 100 extends above inclined slide surface 142 to form a back landsurface 144. Residing adjacent the coating apparatus or hopper 16 is acoating back-up roller 20 about which a web 12 is conveyed. Coatinglayers 108, 116, 124, 132 form a multi-layer composite sheet which formsa coating bead 146 between lip 136 and substrate 12. Typically, thecoating hopper 16 is movable from a non-coating position toward thecoating back-up roller 20 and into a coating position. Although coatingapparatus 16 is shown as having four metering slots, coating dies havinga larger number of metering slots (as many as nine or more) are wellknown and may be used to practice the method of the present invention.

For the purpose of the present invention, the coating fluids for the lowbirefringence protective polymer film are comprised principally of apolymer binder dissolved in an organic solvent. In a particularlypreferred embodiment, the low birefringence protective polymer film is acellulose ester. These are commercially available in a variety ofmolecular weight sizes as well as in the type and degree of alkylsubstitution of the hydroxyl groups on the cellulose backbone. Examplesof cellulose esters include those having acetyl, propionyl and butyrylgroups. Of particular interest is the family of cellulose esters withacetyl substitution known as cellulose acetate. Of these, the fullyacetyl substituted cellulose having a combined acetic acid content ofapproximately 58.0-62.5% is known as triacetyl cellulose (TAC) and isgenerally preferred for preparing cover sheets used in electronicdisplays.

In terms of organic solvents for TAC, suitable solvents, for example,include chlorinated solvents (methylene chloride and 1,2dichloroethane), alcohols (methanol, ethanol, n-propanol, isopropanol,n-butanol, isobutanol, diacetone alcohol, and cyclohexanol), ketones(acetone, methylethyl ketone, methylisobutyl ketone, and cyclohexanone),esters (methyl acetate, ethyl acetate, n-propyl acetate, isopropylacetate, isobutyl acetate, n-butyl acetate, and methylacetoacetate),aromatics (toluene and xylenes) and ethers (1,3-dioxolane,1,2-dioxolane, 1,3-dioxane, 1,4-dioxane, and 1,5-dioxane). In someapplications, small amounts of water may be used. Normally, TACsolutions are prepared with a blend of one or more the aforementionedsolvents. Preferred primary solvents include methylene chloride,acetone, methyl acetate, and 1,3-dioxolane. Preferred co-solvents foruse with the primary solvents include methanol, ethanol, n-butanol. andwater.

Coating formulations may also contain plasticizers. Appropriateplasticizers for TAC films include phthalate esters (dimethylphthalate,dimethoxyethyl phthalate, diethylphthalate, dibutylphthalate,dioctylphthalate, didecylphthalate and butyl octylphthalate), adipateesters (dioctyl adipate), phosphate esters (tricresyl phosphate,biphenylyl diphenyl phosphate, cresyl diphenyl phosphate, octyl diphenylphosphate, tributyl phosphate, and triphenyl phosphate), and glycolicacid esters (triacetin, tributyrin, butyl phthalyl butyl glycolate,ethyl phthalyl ethyl glycolate, and methyl phthalyl ethyl glycolate).Non-aromatic ester plasticizers as described in commonly assignedco-pending U.S. patent application Ser. No. 10/945,305, filed Sep. 20,2004. Plasticizers are normally used to improve the physical andmechanical properties of the final film. In particular, plasticizers areknown to improve the flexibility and dimensional stability of celluloseacetate films. However, plasticizers are also used here as coating aidsin the converting operation to minimize premature film solidification atthe coating hopper and to improve drying characteristics of the wetfilm. In the method of the present invention, plasticizers are used tominimize blistering, curl and delamination of TAC films during thedrying operation. In a preferred embodiment of the present invention,plasticizers are added to the coating fluid at a total concentration ofup to 50% by weight relative to the concentration of polymer in order tomitigate defects in the final TAC film.

As indicated above, a significant aspect of the present invention isthat the protective cover sheet comprising a low birefringenceprotective polymer film, in additional, and one or more relatively thinfunctional layers in addition to the low birefringence protectivepolymer and preferably coated thereon, which functional layer or layersare either adjacent or non-adjacent each other, wherein the one or morefunctional layers comprise at least an adhesion-promoting layer foradhering a poly(vinyl alcohol)-containing film to said low birefringenceprotective polymer film, and wherein at least one of said one or morefunctional layers comprises a UV-absorbing polymer. The UV-absorbingpolymer may be in the adhesion-promoting layer.

Preferably, in one embodiment of the invention, the UV-absorbing polymeris in the form of particles, for example a latex, having a mean particlesize of between 10 and 500 nanometers, more preferably having a meanparticle size of between 10 and 150 nanometers. The UV-absorbing polymercan be water dispersible or soluble in an organic solvent.

Furthermore, the UV-absorbing polymer in one embodiment has a weightaverage molecular weight of greater than 10,000, preferably greater than100,000 to up 5,000,0000 if uncrosslinked, and can optionally becrosslinked.

The UV-absorbing polymer preferably comprise greater than 20 wt. % ofthe functional layer comprising the UV-absorbing polymer.

The UV-absorbing polymer can suitably be selected from polymerscomprising a structural unit having UV absorption capacity. Examplesinclude, but are not limited to, 2-hydroxybenzophenone derivativesincluding 2-hydroxy-4-methoxybenzophenone,2-hydroxy-4-octoxybenzophenone, 2-hydroxy-4-dodecyloxybenzophenone,2-hydroxy-4-methoxy-5-sulfobenzophenone, 2,4-dihydroxybenzophenone,2,2′-dihydroxy-4,4′-dimethoxybenzophenone,bis(2-methoxy-4-hydroxy-5-benzoylphenyl)methane,2-hydroxy-4-acryloyloxybenzophenone,2-hydroxy-4-methacryloyloxybenzophenone,2-hydroxy-4-(2-acryloyloxy)ethoxybenzophenone,2-hydroxy-4-(2-methacryloyloxy)ethoxybenzophenone,2-hydroxy-4-(2-methyl-2-acryloyloxy)ethoxybenzophenone, and the like;2-hydroxybenzotriazole derivatives including2-(2′-hydroxy-5′-methylphenyl)benzotriazole,2-(2′-hydroxy-5′-t-butylphenyl)benzotriazole,2-(2′-hydroxy-5′-3′,5′-di-t-butylphenyl)benzotriazole,2-(2′-hydroxy-3′-t-butyl-5′-methylphenyl)-5-chlorobenzotriazole,2-(2′-hydroxy-3′-5′-di-t-amylphenyl)benzotriazole,2-{2′-hydroxy-3′-(3″,4″,5″,6″-tetrahydrophthalimidomethyl)-5′-methylphenyl}benzotriazole,2,2-methylenebis{4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazole-2-yl)phenol},2-{2′-hydroxy-5′-(methacryloyloxy)phenyl}benzotriazole,2-[2′-hydroxy-5′-(acryloyloxy)phenyl]benzotriazole,2-[2′-hydroxy-3′-t-butyl-5′-(methacryolyoxy)phenyl]benzotriazole,2-[2′-hydroxy-3′-methyl-5′-(acryloyloxy)phenyl]benzotriazole,2-{2′-hydroxy-5′-(methacryloyloxypropyl)phenyl}-5-chlorobenzotriazole,2-[2′-hydroxy-5′-(methacryloyoxyethyl)phenyl]benzotriazole,2-[2′-hydroxy-5′-(acryloyloxyethyl)phenyl]benzotriazole,2-[2′-hydroxy-3′-t-butyl-5′-(methacryloxyethyl)phenyl]benzotriazole,2-[2′-hydroxy-3′-methyl-5′-(acryloyloxyethyl)phenyl]benzotriazole,2-[2′-hydroxy-5′-(methacryloyloxypropyl)phenyl]-5-chlorobenzotrialoe,2-[2′-hydroxy-5′-(acryloyloxybutyl)phenyl]-5-methylbenzotriazole,[2-hydroxy-3-t-butyl-5-(acryloyloxyethoxycarbonylethyl)phenyl]benzotriazole,and the like; salicylic acid derivatives such as phenyl salicylate,p-t-butylphenyl salicylate, p-octylphenyl salicylate, and the like.

Preferably the UV-absorbing polymer has a maximum absorption wavelengthin the range of 300 to 400 nm.

Preferably the UV-absorbing polymer is a copolymer that is the reactionproduct of a mixture of the UV-absorbing vinyl monomer with one or moreco-polymerizable compatible vinyl monomers, wherein the UV-absorbingvinyl monomer comprises a structural unit having UV absorption capacityas described above. The co-polymerizable vinyl monomers can include, forexample, an acrylic acid, an alkylacrylacid (such as methacrylic acid,etc.), an ester or amide derived from an acrylic acid (for example,acrylamide, methacrylamide, n-butylacrylamide, t-butylacrylamide,diacetone acrylamide, methyl acrylate, ethyl acrylate, n-propylacrylate,n-butyl acrylate, t-butyl acrylate, isobutyl acrylate, 2-ethylhexylacrylate, n-octyl acrylate, lauryl acrylate, 2-ethoxyethyl acrylate,2-methoxyethyl acrylate, methyl methacrylate, ethyl methacrylate,n-butyl methacrylate, b-hydroxyl methacrylate, etc.), a vinyl ester (forexample, vinyl acetate, vinyl propionate, vinyl laurate, etc.),acrylonitrile, methacrylonitrile, an aromatic vinyl compound (forexample, styrene and a derivative thereof, for example, vinyl toluene,divinylbenzene, vinyl acetophenone, sulfostyrene, etc.), iraconic acid,citraconic acid, crotonic acid, vinylidene chloride, a vinyl alkyl ether(for example, vinyl ethyl ether, etc.), an ester of maleic acid,N-vinyl-2-pyrrolidone, N-vinylpyridine, 2- or 4-vinylpyridine, etc., ansulfonic acid containing monomers, (for example,acrylamido-2,2′-dimethylpropane sulfonic acid, 2-sulfoethylmethacrylate, 3-sulfopropyl methacylate, etc.). Of these monomers,esters of acrylic acid, esters of methacrylic acid, and aromatic vinylcompounds are preferred. Specific examples of preferred comonomersinclude: butyl acrylate; 2-ethylhexyl acrylate; 2-ethoxyethyl acrylate;2-methoxyethyl acrylate; acrylic acid; methacrylic acid; acrylamide;2-hydroxyethyl acrylate; vinyl acetate; styrene; N-vinyl-2-pyrrolidone;2-sulfoethyl methacrylate and its metal salts; and 2-acrylamido-2-methylpropane sulfonic acid and its metal salts.

In a preferred embodiment of the invention, the polymeric UV-absorbingmaterial comprises the following repeating units:

wherein:

R₁ represents H or CH₃;

R₂ represents H, halogen, alkoxy or a straight chain or branched alkylgroup having from 1 to about 8 carbon atoms;

R₃ represents H, Cl, alkoxy or an alkyl group having from 1 to about 4carbon atoms;

X represents COO, CONH or aryl; and

Y represents an alkylene group having from about 2 to about 10 carbonatoms or (CH₂)_(n)O wherein n is 1 to about 4.

One preferred embodiments of the UV-absorbing polymer is represented byStructure 1 wherein R₁ represents CH₃; R₂ represents H; R₃ represents H;X represents COO; and Y represents CH₂CH₂.

In another preferred embodiment, the UV-absorbing polymer is representedby Structure I wherein R₁ represents H; R₂ represents H; R₃ representsCl; X represents COO; and Y represents CH₂CH₂CH₂.

Specific examples of polymeric UV-absorbing repeating units useful inthe invention include the following:

wherein the groups are as specified in Table 1 below.

TABLE I UV-Absorber R₁ R₂ R₃ X Y UV-1 CH₃ H H COO (CH₂)₂ UV-2 H H Cl COO(CH₂)₃ UV-3 H H H

CH₂O UV-4 CH₃ C(CH₃)₃ H COO (CH₂)₃ UV-5 H CH₃ H CONH CH₂ UV-6 H CH₃ OCH₃CONH CH₂ UV-7 H C(CH₃)₃ Cl CONH CH₂ UV-8 CH₃ H H COO (CH₂)₂OCONH UV-9CH₃ Cl H COO

UV-10 CH₃ H Cl COO (CH₂)₃ UV-11 H H Cl COO (CH₂)₃ UV-12 CH₃ H Cl COO

UV-13 H H Cl COO

UV-14 CH₃ H Cl COO

UV-15 H CH₃ H

CH₂ UV-16 H CH₃ Cl COO (CH₂)₃ UV-17 H CH₃ H COO (CH₂)₂ UV-18 CH₃ H ClCOO (CH₂)₂O UV-19 H H Cl COO (CH₂)₂

The UV absorbing repeating units illustrated in Table 1 above can alsobe polymerized in the presence of two or more comonomers. For example, acombination of ethyl acrylate and acrylamido-2,2′-dimethyl propanesulfonic acid monomers can be copolymerized with UV absorbing repeatingunit UV-1 above. Specific examples of polymeric UV absorbing materialsuseful for this invention are summarized below:

UVL-1: poly-(UV-1)-co-ethylacrylate-co-2-sulfo-1,1-dimethylethylacrylamide, sodium salt (1:1:0.05molar ratio);

UVL-2: poly-(UV-2)-co-ethylacrylate-2-sulfo-1,1-dimethylethylacrylamide, sodium salt (1:1:0.05molar ratio); and

UVL-3: poly-(UV-3)-co-butyl acrylate-co-2-sulfo-1,1dimethylethylacrylamide sodium salt (1:2:0.05 molar ratio).

The polymeric UV-absorbing materials employed in the invention can beused in an amount of from 0.05 to about 4.0 g/m², preferably from about0.20 to about 1.5 g/m².

As indicated above, the functional layer comprising the UV-absorbingpolymer is an adhesion-promoting layer for adhering a poly(vinylalcohol)-containing film to said low birefringence protective polymerfilm. Alternatively, the functional layer comprising the UV-absorbingpolymer is a tie layer between said low birefringence protective polymerfilm and said layer promoting adhesion to poly(vinyl alcohol)-containingfilms. In yet another embodiment, both the adhesion-promoting layer andthe tie layer comprise a UV-absorbing polymer which may be the same ordifferent. As indicated above, the adhesion-promoting layer can furthercomprises a crosslinking compound for the UV absorbing polymer inanother functional layer comprising the UV-absorbing polymer.

The low birefringence protective polymer film comprises one or more lowmolecular weight UV-absorbing non-polymeric or polymeric compounds. Suchcompounds can be non-polymeric or have a molecular weight less than 500,preferably less than 350.

The coating formulation for the low birefringence protective polymerfilm itself may also contain one or more UV absorbing compounds toprovide UV filter element performance and/or act as UV stabilizers forthe low birefringence protective polymer film. Ultraviolet absorbingcompounds are generally contained in the polymer in an amount of 0.01 to20 weight parts based on 100 weight parts of the polymer containing noultraviolet absorber, and preferably contained in an amount of 0.01 to10 weight parts, especially in an amount of 0.05 to 2 weight parts. Anyof the various ultraviolet light absorbing compounds which have beendescribed for use in various polymeric elements may be employed in thepolymeric elements of the invention, such as hydroxyphenyl-s-triazine,hydroxyphenylbenzotriazole, formamidine, or benzophenone compounds. Asdescribed in commonly assigned U.S. Pat. No. 6,872,766, herebyincorporated by reference, the use of dibenzoylmethane ultravioletabsorbing compounds in combination with a second UV absorbing compoundsuch as those listed above have been found to be particularlyadvantageous with respect to providing both a sharp cut off inabsorption between the UV and visible light spectral regions as well asincreased protection across more of the UV spectrum. Additional possibleUV absorbers which may be employed include salicylate compounds such as4-t-butylphenylsalicylate; and[2,2′-thiobis-(4-t-octylphenolate)]n-butylamine nickel(II). Mostpreferred are combinations of dibenzoylmethane compounds withhydroxyphenyl-s-triazine or hydroxyphenylbenzotriazole compounds.

Dibenzoylmethane ultraviolet absorbing compounds which may be employedinclude those of the formula (M-I):

where R1 through R5 are each independently hydrogen, halogen, nitro, orhydroxyl, or further substituted or unsubstituted alkyl, alkenyl, aryl,alkoxy, acyloxy, ester, carboxyl, alkyl thio, aryl thio, alkyl amine,aryl amine, alkyl nitrile, aryl nitrile, arylsulfonyl, or 5-6 memberheterocycle ring groups. Preferably, each of such groups comprises 20 orfewer carbon atoms. Further preferably, R1 through R5 of Formula IV arepositioned in accordance with Formula I-A:

Particularly preferred are compounds of Formula I-A where R1 and R5represent alkyl or alkoxy groups of from 1-6 carbon atoms and R2 throughR4 represent hydrogen atoms.

Representative compounds of Formula (I-A) which may be employed inaccordance with the elements of the invention include the following:

(IV-1): 4-(1,1-dimethylethyl)-4′-methoxydibenzoylmethane (PARSOL 1789);

(IV-2): 4-isopropyl dibenzoylmethane (EUSOLEX 8020); and

(IV-3): dibenzoylmethane (RHODIASTAB 83).

Hydroxyphenyl-s-triazine ultraviolet absorbing compounds which may beused in the elements of the invention, e.g., may be a derivative oftris-aryl-s-triazine compounds as described in U.S. Pat. No. 4,619,956.Such compounds may be represented by Formula II:

wherein X, Y and Z are each aromatic, carbocylic radicals of less thanthree 6-membered rings, and at least one of X, Y and Z is substituted bya hydroxy group ortho to the point of attachment to the triazine ring;and each of R1 through R9 is selected from the group consisting ofhydrogen, hydroxy, alkyl, alkoxy, sulfonic, carboxy, halo, haloalkyl andacylamino. Particularly preferred are hydroxyphenyl-s-triazines of theformula II-A:

wherein R is hydrogen or alkyl of 1-18 carbon atoms.

Hydroxyphenylbenzotriazole compounds which may be used in the elementsof the invention, e.g., may be a derivative of compounds represented byFormula III:

wherein R1 through R5 may be independently hydrogen, halogen, nitro,hydroxy, or further substituted or unsubstituted alkyl, alkenyl, aryl,alkoxy, acyloxy, aryloxy, alkylthio, mono or dialkyl amino, acyl amino,or heterocyclic groups. Specific examples of benzotriazole compoundswhich may be used in accordance with the invention include2-(2′-hydroxy-3′-t-butyl-5′-methylphenyl)-5-chlorobenzotriazole;2-(2′-hydroxy-3′,5′-di-t-amylphenyl)benzotriazole; octyl5-tert-butyl-3-(5-chloro-2H-benzotriazole-2-yl)-4-hydroxybenzenepropionate;2-(hydroxy-5-t-octylphenyl)benzotriazole;2-(2′-hydroxy-5′-methylphenyl)benzotriazole;2-(2′-hydroxy-3′-dodecyl-5′-methylphenyl)benzotriazole; and2-(2′-hydroxy-3′,5′-di-t-butylphenyl)-5-chlorobenzotriazole.

Formamidine ultraviolet absorbing compounds which may be used in theelements of the invention, e.g., may be a formamidine compound asdescribed in U.S. Pat. No. 4,839,405. Such compounds may be representedby Formula IV or Formula V:

wherein R1 is an alkyl group containing 1 to about 5 carbon atoms; Y isa H, OH, Cl or an alkoxy group; R2 is a phenyl group or an alkyl groupcontaining 1 to about 9 carbon atoms; X is selected from the groupconsisting of H, carboalkoxy, alkoxy, alkyl, dialkylamino and halogen;and Z is selected from the group consisting of H, alkoxy and halogen;

wherein A is —COOR, —COOH, —CONR′R″, —NR′COR, —CN, or a phenyl group;and wherein R is an alkyl group of from 1 to about 8 carbon atoms; R′and R″ are each independently hydrogen or lower alkyl groups of from 1to about 4 carbon atoms. Specific examples of formamidine compoundswhich may be used in accordance with the invention include thosedescribed in U.S. Pat. No. 4,839,405, and specifically4-[[(methylphenylamino)methylene]amino]-ethyl ester.

Benzophenone compounds which may be used in the elements of theinvention, e.g., may include 2,2′-dihydroxy-4,4′dimethoxybenzophenone,2-hydroxy-4-methoxybenzophenone, and2-hydroxy-4-n-dodecyloxybenzophenone.

Coating formulations may also contain surfactants as coating aids tocontrol artifacts related to flow after coating. Artifacts created byflow after coating phenomena include mottle, repellencies, orange-peel(Bernard cells), and edge-withdraw. Surfactants used control flow aftercoating artifacts include siloxane and fluorochemical compounds.Examples of commercially available surfactants of the siloxane typeinclude: (1) Polydimethylsiloxanes such as DC200 Fluid from Dow Corning;(2) Poly(dimethyl, methylphenyl)siloxanes such as DC510 Fluid from DowCorning; (3) Polyalkyl substituted polydimethysiloxanes such as DC190and DC1248 from Dow Corning as well as the L7000 SILWET series (L7000,L7001, L7004 and L7230) from Union Carbide; and (4) Polyalkylsubstituted poly(dimethyl, methylphenyl)siloxanes such as SF1023 fromGeneral Electric. Examples of commercially available fluorochemicalsurfactants include: (1) Fluorinated alkyl esters such as the FLUORADseries (FC430 and FC431) from the 3M Corporation; (2) Fluorinatedpolyoxyethylene ethers such as the ZONYL series (FSN, FSN100, FSO,FSO100) from DuPont; (3) Acrylate:polyperfluoroalkyl ethylacrylates suchas the F series (F270 and F600) from NOF Corporation; and (4)Perfluoroalkyl derivatives such as the SURFLON series (S383, S393, andS8405) from the Asahi Glass Company. In the method of the presentinvention, surfactants are generally of the non-ionic type. In apreferred embodiment of the present invention, non-ionic compounds ofeither the siloxane or fluorinated type are added to the uppermostlayers.

In terms of surfactant distribution, surfactants are most effective whenpresent in the uppermost layers of the multi-layer coating. In theuppermost layer, the concentration of surfactant is preferably0.001-1.000% by weight and most preferably 0.010-0.500%. In addition,lesser amounts of surfactant may be used in the second uppermost layerto minimize diffusion of surfactant into the lowermost layers. Theconcentration of surfactant in the second uppermost layer is preferably0.000-0.200% by weight and most preferably between 0.000-0.100% byweight. Because surfactants are only necessary in the uppermost layers,the overall amount of surfactant remaining in the final dried film issmall.

Although surfactants are not required to practice the method of thecurrent invention, surfactants do improve the uniformity of the coatedfilm. In particular, mottle non-uniformities are reduced by the use ofsurfactants. In transparent cellulose acetate films, mottlenon-uniformities are not readily visualized during casual inspection. Tovisualize mottle artifacts, organic dyes may be added to the uppermostlayer to add color to the coated film. For these dyed films,non-uniformities are easy to see and quantify. In this way, effectivesurfactant types and levels may be selected for optimum film uniformity.

As an alternative to the exemplary coating method and apparatus of FIG.3 for making the low birefringence protective polymer film, a castingmethod and apparatus can be used. Turning now to FIG. 4 there is shown aschematic of an exemplary casting and drying system suitable forpreparing the cover sheets of the present invention. A viscous dopecomprising a low birefringence protective polymer is delivered through afeed line 200 to an extrusion hopper 202 from a pressurized tank 204 bya pump 206. The dope is cast onto a highly polished metal drum 208located within a first drying section 210 of the drying oven 212. Thecast polymer film 214 is allowed to partially dry on the moving metaldrum 208 and is then peeled from the metal drum 208. The cast polymerfilm 214 is then conveyed to a final drying section 216 to remove theremaining solvent. The final dried low birefringence protective polymerfilm 218 is then wound into rolls at a wind-up station 220. The castpolymer film typically has a thickness in the range of from 40 to 200μm.

Coating methods such as illustrated in FIG. 3 are distinguished fromcasting methods such as illustrated in FIG. 4 by the process stepsnecessary for each technology. These process steps in turn affect anumber of tangibles such as fluid viscosity, converting aids,substrates, and hardware that are unique to each method. In general,coating methods involve application of dilute low viscosity liquids tothin flexible substrates, evaporating the solvent in a drying oven, andwinding the dried film/substrate composite into rolls. In contrast,casting methods involve applying a concentrated viscous dope to a highlypolished metal drum or band, partially drying the wet film on the metalsubstrate, stripping the partially dried film from the substrate,removing additional solvent from the partially dried film in a dryingoven, and winding the dried film into rolls. In terms of viscosity,coating methods require very low viscosity liquids of less than 5,000cp. In the present invention the viscosity of the coated liquids willgenerally be less than 2000 cp and most often less than 1500 cp.Moreover, in the coating method the viscosity of the lowermost layer ispreferred to be less than 200 cp. and most preferably less than 100 cp.for high speed coating application. In contrast, casting methods requirehighly concentrated dopes with viscosity on the order of 10,000-100,000cp for practical operating speeds. In terms of converting aids, coatingmethods generally involve the use of surfactants as converting aids tocontrol flow after coating artifacts such as mottle, repellencies,orange peel, and edge withdraw. In contrast, casting methods do notrequire surfactants. Instead, converting aids are only used to assist inthe stripping operation in casting methods. For example, n-butanol issometimes used as a converting aid in casting TAC films to facilitatestripping of the TAC film from the metal drum. In terms of substrates,coating methods generally utilize thin (10-250 μm) flexible supports. Incontrast, casting methods employ thick (1-100 mm), continuous, highlypolished metal drums or rigid bands. As a result of these differences inprocess steps, the hardware used in coating is conspicuously differentfrom those used in casting as can be seen by a comparison of theschematics shown in FIGS. 1 and 4, respectively.

The invention is also directed to a guarded cover sheet compositecomprising a carrier substrate and a multilayer protective cover sheetcomprising a low birefringence protective polymer film and one or morefunctional layers, which layers are either adjacent or non-adjacent,wherein the one or more functional layers comprise at least anadhesion-promoting layer, for adhering a poly(vinyl alcohol)-containingfilm, on the same side of said carrier substrate as the lowbirefringence polymer film to said low birefringence protective polymerfilm, and wherein at least one of said one or more functional layerscomprise a UV-absorbing polymer which may be the same or different thanthe adhesion-promoting layer. The preparation of the cover sheet or theguarded cover sheet composite of the present invention may also includethe step of coating over a previously prepared (by coating or castingprocess) film. For example, the coating and drying system 10 shown inFIGS. 1 and 2 may be used to apply a second film or multi-layer film toan existing low birefringence protective polymer film or cover sheetcomposite. If the film or cover sheet composite is wound into rollsbefore applying the subsequent coating, the process is called amulti-pass coating operation. If coating and drying operations arecarried out sequentially on a machine with multiple coating stations anddrying ovens, then the process is called a tandem coating operation. Inthis way, thick low birefringence protective polymer films may beprepared at high line speeds without the problems associated with theremoval of large amounts of solvent from a very thick wet film.Alternatively, many different cover sheet configurations having variouscombinations of auxiliary layers applied via a tandem or multi-passcoating operation may be prepared. Moreover, the practice of multi-passor tandem coating also has the advantage of minimizing other artifactssuch as streak severity, mottle severity, and overall filmnon-uniformity.

Turning next to FIGS. 5 through 8, there are presented cross-sectionalillustrations showing various cover sheet and guarded cover sheetcomposite configurations possible with the present invention. FIG. 5shows a cover sheet 189 having lowermost layer 186, intermediate layers187 and 188, and uppermost layer 190. In this illustration, layer 186could be a layer promoting adhesion to PVA, 187 could be a tie layer,layer 188 could be a low birefringence protective polymer film, andlayer 190 could be an auxiliary layer such as a viewing anglecompensation layer, moisture barrier layer, abrasion resistant layer, orother type of auxiliary layer, for example. The cover sheet may beprepared by conventional casting methods or by coating methods employinga carrier substrate as described hereinabove.

In FIG. 6, a guarded cover sheet composite 151 comprising a three-layercover sheet 171 having lower-most layer 162, intermediate layer 164, anduppermost layer 168 is shown partially peeled from a carrier substrate170. In this illustration, layer 162 could be a layer promoting adhesionto PVA, layer 164 could be a tie layer, and layer 168 could be a lowbirefringence protective polymer film. Layers 162, 164, and 168 may beformed either by applying and drying three separate liquid layers on thecarrier substrate 170 or by simultaneously applying two or all three ofthe layers and then drying those simultaneously applied layers in asingle drying operation.

In a preferred embodiment, the layer promoting adhesion to PVA is coatedand dried separately from the tie layer and low birefringence protectivepolymer film using a water-based coating formulation. When a cover sheet171 is prepared by coating onto a carrier substrate 170 as illustratedin FIG. 6, it is generally preferred that the layer promoting adhesionto PVA is coated onto the carrier substrate 170 and then dried, prior toapplication of the low birefringence protective polymer film. Auxiliarylayers may be applied either simultaneously with the low birefringenceprotective polymer film or in a subsequent coating and drying operation.

FIG. 7 illustrates another guarded cover sheet composite 153 comprisinga cover sheet 173 that is comprised of, for example, fourcompositionally discrete layers including a lowermost layer 162 nearestto the carrier support 170, two intermediate layers 164 and 166, and anuppermost layer 168. FIG. 7 also shows that the entire multiple layercover sheet 173 may be peeled from the carrier substrate 170. In thisillustration, layer 162 could be a layer promoting adhesion to PVA,layer 164 could be a tie layer, layer 166 could be a low birefringenceprotective polymer film, and layer 168 could be an auxiliary layer suchas an abrasion resistant layer, for example.

FIG. 8 illustrates a further guarded cover sheet composite 159comprising a cover sheet 179 that is comprised of, for example, fourcompositionally discrete layers including a lowermost layer 174 nearestto the carrier substrate 182, two intermediate layers 176 and 178, andan uppermost layer 180. The carrier substrate 182 has been treated witha release layer 184 to modify the adhesion between the cover sheetlowermost layer 174 and substrate 182. Release layer 184 may becomprised of a number of polymeric materials such as polyvinylbutyrals,cellulosics, polyacrylates, polycarbonates, andpoly(acrylonitrile-co-vinylidene chloride-co-acrylic acid). The choiceof materials used in the release layer may be optimized empirically bythose skilled in the art.

FIGS. 5 through 8 serve to illustrate some of the guarded cover sheetcomposites that may be constructed based on the detailed teachingsprovided hereinabove, they are not intended to be exhaustive of allpossible variations of the invention. One skilled in the art couldconceive of many other layer combinations that would be useful asguarded cover sheet composites for use in the preparation of polarizerplates for displays.

Turning now to FIG. 9, a schematic representation of a method tofabricate a polarizer plate from guarded cover sheet composites of theinvention is illustrated. Guarded cover sheet composite 151 (see FIG. 6)comprising cover sheet 171 and carrier substrate 170 and guarded coversheet composite 153 (see FIG. 7) comprising cover sheet 173 and carriersubstrate 170 are supplied from supply rolls 232 and 234, respectively.A PVA dichroic film is supplied from supply roll 236. Prior to enteringa lamination nip between opposing pinch rollers 242 and 244, the carriersubstrate 170 is peeled from guarded cover sheet composites 151 and 153to expose a lowermost layer (in the case of FIGS. 6 and 7, this is layer162, which for the purpose of example is the layer promoting adhesion toPVA). The peeled carrier sheet 170 is wound into rolls at take-up rolls240. A glue solution may be optionally applied to both sides of the PVAdichroic film or to the lower-most layer of cover sheets 171 and 173prior to the sheets and film entering the nip between pinch rollers 242and 244. Cover sheets 171 and 173 are then laminated to either side ofPVA dichroic film with the application of pressure (and, optionally,heat) between the opposing pinch rollers 242 and 244 resulting in thepolarizer plate 250 in sheet form. Polarizer plate 250 may then be driedby heating and wound into rolls until needed. Depending on theparticular layer configuration for the guarded cover sheet compositesemployed, a wide variety of polarizer plates having cover sheets withvarious combinations of auxiliary layers may be fabricated.

For cover sheets of the invention wherein a low birefringence protectivepolymer film is prepared by a conventional casting process (wherein apolymer dope is case onto a continuous metal wheel or drum and thenpeeled prior to completion of the drying process) and the tie layer andlayer promoting adhesion to PVA are applied in a subsequent coatingoperation, the method of fabricating polarizing plates is simplifiedcompared to that represented in FIG. 9. In this case, since a carriersubstrate is not employed, the step of peeling and winding the carriersubstrate as shown in FIG. 9 is not necessary. Instead, the cover sheet,which is preferably supplied in roll form, merely needs to be unwoundand supplied to the lamination nip formed between a pair of pinchrollers that are analogous to rollers 242 and 244 shown FIG. 9. Asbefore, a glue solution may be optionally applied to both sides of thePVA dichroic film or to the layers promoting adhesion to PVA prior tothe cover sheets and film entering the nip between the pinch rollers.

The cover sheet is laminated to the PVA dichroic film such that thelayer promoting adhesion to PVA is on the side of the cover sheet thatcontacts the PVA dichroic film. The glue solution useful for laminatingthe cover film and the PVA dichroic film is not particularly limited, acommonly employed example is a water/alcohol solution containing adissolved polymer such as PVA or its derivatives and a boron compoundsuch as boric acid. Alternatively, the solution may be free orsubstantially free of dissolved polymer and comprise a reagent thatcrosslinks PVA. The reagent may crosslink PVA either ionically orcovalently or a combination of both types of reagents may be used.Appropriate crosslinking ions include but are not limited to cationssuch as calcium, magnesium, barium, strontium, boron, beryllium,aluminum, iron, copper, cobalt, lead, silver, zirconium, and zinc ions.Boron compounds such as boric acid and zirconium compounds such aszirconium nitrate or zirconium carbonate are particularly preferred.Examples of covalent crosslinking reagents include polycarboxylic acidsor anhydrides; polyamines; epihalohydrins; diepoxides; dialdehydes;diols; carboxylic acid halides, ketenes and like compounds. The amountof the solution applied onto the films can vary widely depending on itscomposition. For example, a wet film coverage as low as 1 cc/m² and ashigh as 100 cc/m² are possible. Low wet film coverages are desirable toreduce the drying time needed.

Low birefringence protective polymer films suitable for use in thepresent invention comprise polymeric materials having low IntrinsicBirefringence Δn_(int) that form high clarity films with high lighttransmission (i.e., >85%). Preferably, the low birefringence protectivepolymer film has in-plane birefringence, Δn_(in) of less than about1×10⁻⁴ and an out-of-plane birefringence, Δn_(th) of from 0.005 to−0.005.

Exemplary polymeric materials for use in the low birefringenceprotective polymer films of the invention include cellulose esters(including triacetyl cellulose (TAC), cellulose diacetate, celluloseacetate butyrate, cellulose acetate propionate), polycarbonates (such asLEXAN available from General Electric Corp.,bisphenol-A-trimethylcyclohexane-polycarbonate,bisphenol-A-phthalate-polycarbonate), polysulfones (such as UDELavailable from Amoco Performance Products Inc.), polyacrylates, andcyclic olefin polymers (such as ARTON available from JSR Corp., ZEONEXand ZEONOR available from Nippon Zeon, TOPAS supplied by Ticona), amongothers. Preferably, the low birefringence protective polymer film of theinvention comprises TAC, polycarbonate, or cyclic olefin polymers duetheir commercial availability and excellent optical properties.

The low birefringence protective polymer film has a thickness from about5 to 200 micrometers, preferably from about 5 to 80 micrometers and mostpreferably from about 20 to 80 micrometers. Films having thickness of 20to 80 micrometers are most preferred due to cost, handling, and theability to fabricate thinner polarizer plates. In a preferred embodimentof the current invention, polarizer plates assembled from cover sheetsof the invention have a total thickness of less than 120 micrometers,and most preferably less than 80 micrometers.

The layer promoting adhesion to PVA can comprise one or morewater-soluble polymers suitable for the purpose of the present inventionincluding, for example, both synthetic and natural polymers. Naturallyoccurring polymers include proteins, protein derivatives, cellulosederivatives (e.g. cellulose esters), polysaccharides, casein, and thelike, and synthetic polymers include poly(vinyl lactams), acrylamidepolymers, polyvinyl alcohol and its derivatives, hydrolyzed polyvinylacetates, polymers of alkyl and sulfoalkyl acrylates and methacrylates,polyamides, polyvinyl pyridine, acrylic acid polymers, maleic anhydridecopolymers, polyalkylene oxide, methacrylamide copolymers, polyvinyloxazolidinones, maleic acid copolymers, vinyl amine copolymers,methacrylic acid copolymers, acryloyloxyalkyl sulfonic acid copolymers,vinyl imidazole copolymers, vinyl sulfide copolymers, homopolymer orcopolymers containing styrene sulfonic acid, and the like. The mostpreferred polymers are polyvinyl alcohol and its derivatives.

Particularly preferred polyvinyl alcohol polymers have a degree ofhydrolysis of between 75 and 100% and have a weight average molecularweight of greater than 10,000.

Hydrophobic polymer particles useful in the adhesion-promoting layerinclude water dispersible polymers and polymer latexes. In order topromote interaction with PVA, the hydrophobic polymer particlespreferably contain hydrogen bonding groups, which includes hydroxyl,carboxyl, amino, or sulfonyl moieties. Suitable hydrophobic polymerparticles can comprise addition-type polymers and copolymers (includinginterpolymers) prepared from ethylenically unsaturated monomers such asacrylates including acrylic acid, methacrylates including methacrylicacid, acrylamides and methacrylamides, itaconic acid and its half estersand diesters, styrenes including substituted styrenes, acrylonitrile andmethacrylonitrile, vinyl acetates, vinyl ethers, vinyl and vinylidenehalides, and olefins. In addition, crosslinking and graft-linkingmonomers such as 1,4-butyleneglycol methacrylate, trimethylolpropanetriacrylate, allyl methacrylate, diallyl phthalate, divinyl benzene, andthe like may be used. Other suitable polymer dispersions arepolyurethane dispersions or polyesterionomer dispersions,polyurethane/vinyl polymer dispersions, and fluoropolymer dispersions.Preferably, polymers for use in the polymer particles of the inventionhave a weight average molecular weight of greater than about 10,000 anda glass transition temperature (Tg) of less than about 25° C. Ingeneral, high molecular weight, low Tg polymer particles provideimproved adhesion of the layer to both PVA dichroic films and the lowbirefringence polymer film.

Dispersions of hydrophobic polymer particles having a mean particle sizein the range of from 10 nanometers to 1 micron, preferably from 10 to500 nanometers, and most preferably from 10 to 200 nanometers can beemployed. Preferably, the polymer particles comprise between 10 and 40weight % of the layer promoting adhesion to PVA.

The adhesion-promoting layer is highly transparent and, preferably, hasa light transmission of greater than 90%, preferably 95%.

The layer promoting adhesion to PVA may also contain a crosslinkingagent. Crosslinking agents useful for the practice of the inventioninclude any compounds that are capable of reacting with reactivemoieties present on the water soluble polymer and/or polymer particles.Such crosslinking agents include aldehydes and related compounds,pyridiniums, olefins such as bis(vinylsulfonyl methyl)ether,carbodiimides, epoxides, triazines, polyfunctional aziridines,methoxyalkyl melamines, polyisocyanates, and the like. These compoundscan be readily prepared using the published synthetic procedure orroutine modifications that would be readily apparent to one skilled inthe art of synthetic organic chemistry. Additional crosslinking agentsthat may also be successfully employed in the layer promoting adhesionto PVA include multivalent metal ions such as zinc, calcium, zirconiumand titanium.

The layer promoting adhesion to PVA is typically applied at a driedcoating weight of 5 to 300 mg/ft² (50 to 3000 mg/m²), preferably 5 to100 mg/ft² (50 to 1000 mg/m²). The layer may be on either side of thecover sheet relative to the low birefringence film. For the guardedcover sheet composites of the invention, preferably, the layer promotingadhesion to PVA is between the carrier substrate and the lowbirefringence film. Most preferably, the layer promoting adhesion to PVAis applied directly onto the carrier substrate or onto a subbing layeron the carrier substrate. The layer promoting adhesion to PVA may becoated in a separate coating application or it may be appliedsimultaneously with one or more other layers.

In order to provide good wetting by the water-based glues that may beemployed to laminate the cover sheets of the invention to PVA dichroicfilms it is preferred that the PVA adhesion-promoting layer of theinvention has a water contact angle of less than 20°. Theadhesion-promoting layer of the invention also preferably has a waterswell of between 20 and 1000% to promote good contact and perhapsintermixing of the adhesion-promoting layer with the glue and/or PVAdichroic film.

One embodiment of an adhesion-promoting layer is disclosed in U.S.patent application Ser. No. 10/994,710, filed Nov. 22, 2004.

A tie-layer can be applied to the low birefringence protective polymerfilm before the adhesion-promoting layer as disclosed in commonlyassigned copending U.S. patent application Ser. No. 10/994,711, filedNov. 22, 2004, hereby incorporated by reference.

Carrier substrates suitable for the use in the present invention includepolyethylene terephthalate (PET), polyethylene naphthalate (PEN),polycarbonate, polystyrene, and other polymeric films. Additionalsubstrates may include paper, laminates of paper and polymeric films,glass, cloth, aluminum and other metal supports. Preferably, the carriersubstrate is a polyester film comprising polyethylene terephthalate(PET) or polyethylene naphthalate (PEN). The thickness of the carriersubstrate is about 20 to 200 micrometers, typically about 40 to 100micrometers. Thinner carrier substrates are desirable due to both costand the weight per roll of guarded cover sheet composite. However,carrier substrates less than about 20 micrometers may not providesufficient dimensional stability or protection for the cover sheet.

The carrier substrate may be coated with one or more subbing layers ormay be pretreated with electrical discharge devices to enhance thewetting of the substrate by coating solutions. Since the cover sheetmust ultimately be peeled from the carrier substrate the adhesionbetween cover sheet and substrate is an important consideration. Subbinglayers and electrical discharge devices may also be employed to modifythe adhesion of the cover sheet to the carrier substrate. Subbing layersmay therefore function as either primer layers to improve wetting orrelease layers to modify the adhesion of the cover sheet to thesubstrate. The carrier substrate may be coated with two subbing layers,the first layer acting as a primer layer to improve wetting and thesecond layer acting as a release layer. The thickness of the subbinglayer is typically 0.05 to 5 micrometers, preferably 0.1 to 1micrometers.

Cover sheet/substrate composites having poor adhesion might be prone toblister after application of a second or third wet coating in amulti-pass operation. To avoid blister defects, adhesion should begreater than about 0.3 N/m between the first-pass layer of the coversheet and the carrier substrate. As already mentioned, the level ofadhesion may be modified by a variety of web treatments includingvarious subbing layers and various electronic discharge treatments.However, excessive adhesion between the cover sheet and substrate isalso undesirable since the cover sheet may be damaged during subsequentpeeling operations. In particular, cover sheet/substrate compositeshaving too great an adhesive force may peel poorly. The maximum adhesiveforce that allows acceptable peel behavior is dependent on the thicknessand tensile properties of the cover sheet. Typically, an adhesive forcebetween the cover sheet and the substrate greater than about 300 N/m maypeel poorly. Cover sheets peeled from such excessively well-adheredcomposites exhibit defects due to tearing of the cover sheet and/or dueto cohesive failure within the sheet. In a preferred embodiment of thepresent invention, the adhesion between the cover sheet and the carriersubstrate is less than 250 N/m. Most preferably, the adhesion betweenthe cover sheet and the carrier substrate is between 0.5 and 25 N/m.

In a preferred embodiment of the invention, the carrier substrate is apolyethylene terephthalate film having a first subbing layer (primerlayer) comprising a vinylidene chloride copolymer and second subbinglayer (release layer) comprising polyvinyl butyral. In another preferredembodiment of the invention the carrier substrate is polyethyleneterephthalate film that has been pretreated with a corona dischargeprior to application of the cover sheet.

Substrates may also have functional layers such as antistatic layerscontaining various polymer binders and conductive addenda in order tocontrol static charging and dirt and dust attraction. The antistaticlayer may be on either side of the carrier substrate, preferably it ison the side of the carrier substrate opposite to the cover sheet.

On the side of the substrate opposite to the cover sheet a backing layermay also be employed in order to provide a surface having appropriateroughness and coefficient of friction for good winding and conveyancecharacteristics. In particular, the backing layer comprises a polymericbinder such as a polyurethane or acrylic polymer containing mattingagent such a silica or polymeric beads. The matting agent helps toprevent the sticking of the front side of the guarded cover sheetcomposite to the backside during shipping and storage. The backing layermay also comprise a lubricant to provide a coefficient of friction ofabout 0.2 to 0.4. Typical lubricants include for example (1) liquidparaffin and paraffin or wax like materials such as carnauba wax,natural and synthetic waxes, petroleum waxes, mineral waxes and thelike; (2) higher fatty acids and derivatives, higher alcohols andderivatives, metal salts of higher fatty acids, higher fatty acidesters, higher fatty acid amides, polyhydric alcohol esters of higherfatty acids, etc., disclosed in U.S. Pat. Nos. 2,454,043; 2,732,305;2,976,148; 3,206,311; 3,933,516; 2,588,765; 3,121,060; 3,502,473;3,042,222; and 4,427,964, in British Patents 1,263,722; 1,198,387;1,430,997; 1,466,304; 1,320,757; 1,320,565; and 1,320,756; and in GermanPatents 1,284,295 and 1,284,294; and (3) perfluoro- or fluoro- orfluorochloro-containing materials, which includepoly(tetrafluoroethylene), poly(trifluorochloroethylene),poly(vinylidene fluoride, poly(trifluorochloroethylene-co-vinylchloride), poly(meth)acrylates or poly(meth)acrylamides containingperfluoroalkyl side groups, and the like. However for lasting lubricitya polymerizable lubricant such as ADDITIVE 31, a methacryloxy-functionalsilicone polyether copolymer (from Dow Corning Corp.) is preferred.

In a preferred embodiment the guarded cover sheet composite comprises astrippable, protection layer on the surface of the cover sheet oppositeto the carrier substrate. The strippable, protection layer may beapplied by coating the layer or it may be applied by adhesively adheringor by electrostatically adhering, a preformed protection layer.Preferably, the protection layer is a transparent polymer layer. In oneparticular embodiment, the protection layer is a low birefringence layerthat allows optical inspection of the cover sheet without the need toremove the protection layer. Particularly useful polymers for use in theprotection layer include: cellulose esters, acrylics, polyurethanes,polyesters, cyclic olefin polymers, polystyrene, polyvinyl butyral,polycarbonate, and others. When a preformed protection layer is used, itis preferably a layer of polyester, polystyrene, or polyolefin film.

The strippable, protection layer is typically 5 to 100 micrometers inthickness. Preferably, the protection layer is 20 to 50 micrometersthick to insure adequate resistance to scratch and abrasion and provideeasy handling during removal of the protection layer.

When the strippable, protection layer is applied by coating methods itmay be applied to an already coated and dried cover sheet or theprotection layer may be coated simultaneously with one or more layerscomprising the cover sheet.

When the strippable, protection layer is a preformed layer it may have apressure sensitive adhesive layer on one surface that allows theprotection layer to be adhesively laminated to the guarded cover sheetcomposite using conventional lamination techniques. Alternatively, thepreformed protection layer may be applied by generating an electrostaticcharge on a surface of the cover sheet or the preformed protection layerand then bringing the two materials into contact in a roller nip. Theelectrostatic charge may be generated by any known electric chargegenerator, e.g., a corona charger, a tribocharger, conducting highpotential roll charge generator or contact charger, a static chargegenerator, and the like. The cover sheet or the preformed protectionlayer may be charged with a DC charge or a DC charge followed by an ACcharge in order to create an adequate level of charge adhesion betweenthe two surfaces. The level of electrostatic charge applied to provide asufficient bond between the cover sheet and the preformed protectionlayer is at least more than 50 volts, preferably at least more than 200volts. The charged surface of the cover sheet or the protection layerhas a resistivity of at least about 10¹² Ω/square, preferably at leastabout 10¹⁶ Ω/square in order to insure that the electrostatic charge islong lasting.

Each protective cover sheet may have various auxiliary layers that arenecessary to improve the performance of a Liquid Crystal Display. LiquidCrystal Displays typically employ two polarizer plates, one on each sideof the liquid crystal cell. Each polarizer plate, in turn, employs twocover sheets, one on each side of the PVA dichroic film. These coversheets may be different, for example, contain a different subset ofpossible auxiliary layers.

Useful auxiliary layers employed in the cover sheets of the inventioncan, for example, include: abrasion resistant hardcoat layer, antiglarelayer, anti-smudge layer or stain-resistant layer, antireflection layer,low reflection layer, antistatic layer, viewing angle compensationlayer, and moisture barrier layer. Typically, the cover sheet closest tothe viewer contains one or more of the following auxiliary layers: theabrasion resistant layer, anti-smudge or stain-resistant layer,antireflection layer, and antiglare layer. One or both of the coversheets closest to the liquid crystal cell typically contain a viewingangle compensation layer. Any or all of the four cover sheets employedin the LCD may optionally contain an antistatic layer and a moisturebarrier layer.

The cover sheets of the invention may contain an abrasion resistantlayer on the opposite side of the low birefringence protective polymerfilm to the layer promoting adhesion to PVA.

Particularly effective abrasion resistant layers for use in the presentinvention comprise radiation or thermally cured compositions, andpreferably the composition is radiation cured. Ultraviolet (UV)radiation and electron beam radiation are the most commonly employedradiation curing methods. UV curable compositions are particularlyuseful for creating the abrasion resistant layer of this invention andmay be cured using two major types of curing chemistries, free radicalchemistry and cationic chemistry. Acrylate monomers (reactive diluents)and oligomers (reactive resins and lacquers) are the primary componentsof the free radical based formulations, giving the cured coating most ofits physical characteristics. Photo-initiators are required to absorbthe UV light energy, decompose to form free radicals, and attack theacrylate group C═C double bond to initiate polymerization. Cationicchemistry utilizes cycloaliphatic epoxy resins and vinyl ether monomersas the primary components. Photo-initiators absorb the UV light to forma Lewis acid, which attacks the epoxy ring initiating polymerization. ByUV curing is meant ultraviolet curing and involves the use of UVradiation of wavelengths between 280 and 420 nm preferably between 320and 410 nm.

Examples of UV radiation curable resins and lacquers usable for theabrasion resistant layer useful in this invention are those derived fromphoto polymerizable monomers and oligomers such as acrylate andmethacrylate oligomers (the term “(meth)acrylate” used herein refers toacrylate and methacrylate), of polyfunctional compounds, such aspolyhydric alcohols and their derivatives having (meth)acrylatefunctional groups such as ethoxylated trimethylolpropanetri(meth)acrylate, tripropylene glycol di(meth)acrylate,trimethylolpropane tri(meth)acrylate, diethylene glycoldi(meth)acrylate, pentaerythritol tetra(meth)acrylate, pentaerythritoltri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, 1,6-hexanedioldi(meth)acrylate, or neopentyl glycol di(meth)acrylate and mixturesthereof, and acrylate and methacrylate oligomers derived fromlow-molecular weight polyester resin, polyether resin, epoxy resin,polyurethane resin, alkyd resin, spiroacetal resin, epoxy acrylates,polybutadiene resin, and polythiol-polyene resin, and the like andmixtures thereof, and ionizing radiation-curable resins containing arelatively large amount of a reactive diluent. Reactive diluents usableherein include monofunctional monomers, such as ethyl (meth)acrylate,ethylhexyl (meth)acrylate, styrene, vinyltoluene, andN-vinylpyrrolidone, and polyfunctional monomers, for example,trimethylolpropane tri(meth)acrylate, hexanediol (meth)acrylate,tripropylene glycol di(meth)acrylate, diethylene glycoldi(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritolhexa(meth)acrylate, 1,6-hexanediol di(meth)acrylate, or neopentyl glycoldi(meth)acrylate.

Among others, in the present invention, conveniently used radiationcurable lacquers, for use in abrasion resistant layers, include urethane(meth)acrylate oligomers. These are derived from reacting diisocyanateswith an oligo(poly)ester or oligo(poly)ether polyol to yield anisocyanate terminated urethane. Subsequently, hydroxy terminatedacrylates are reacted with the terminal isocyanate groups. Thisacrylation provides the unsaturation to the ends of the oligomer. Thealiphatic or aromatic nature of the urethane acrylate is determined bythe choice of diisocyanates. An aromatic diisocyanate, such as toluenediisocyanate, will yield an aromatic urethane acrylate oligomer. Analiphatic urethane acrylate will result from the selection of analiphatic diisocyanate, such as isophorone diisocyanate or hexyl methyldiisocyanate. Beyond the choice of isocyanate, polyol backbone plays apivotal role in determining the performance of the final the oligomer.Polyols are generally classified as esters, ethers, or a combination ofthese two. The oligomer backbone is terminated by two or more acrylateor methacrylate units, which serve as reactive sites for free radicalinitiated polymerization. Choices among isocyanates, polyols, andacrylate or methacrylate termination units allow considerable latitudein the development of urethane acrylate oligomers. Urethane acrylates,like most oligomers, are typically high in molecular weight andviscosity. These oligomers are multifunctional and contain multiplereactive sites. Because of the increased number of reactive sites, thecure rate is improved and the final product is cross-linked. Theoligomer functionality can vary from 2 to 6.

Among others, conveniently used radiation curable resins, for use inabrasion resistant layers, also include polyfunctional acrylic compoundsderived from polyhydric alcohols and their derivatives such as mixturesof acrylate derivatives of pentaerythritol such as pentaeryhritoltetraacrylate and pentaerythritol triacrylate functionalized aliphaticurethanes derived from isophorone diisocyanate. Some examples ofurethane acrylate oligomers used in the practice of this invention thatare commercially available include oligomers from Sartomer Company(Exton, Pa.). An example of a resin that is conveniently used in thepractice of this invention is CN 968 from Sartomer Company.

In one embodiment, an abrasion resistant layer includes a photopolymerization initiator, such as an acetophenone compound, abenzophenone compound, Michler's benzoyl benzoate, α-amyloxime ester, ora thioxanthone compound and a photosensitizer such as n-butyl amine,triethylamine, or tri-n-butyl phosphine, or a mixture thereof isincorporated in the ultraviolet radiation curing composition. In thepresent invention, conveniently used initiators are 1-hydroxycyclohexylphenyl ketone and 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropanone-1.

The abrasion resistant layer is typically applied after coating anddrying the low birefringence protective polymer film. The abrasionresistant layer of this invention is applied as a coating compositionthat typically also includes organic solvents. Preferably theconcentration of organic solvent is 1-99% by weight of the total coatingcomposition.

Examples of solvents employable for coating the abrasion resistant layerof this invention include solvents such as methanol, ethanol, propanol,butanol, cyclohexane, heptane, toluene and xylene, esters such as methylacetate, ethyl acetate, propyl acetate and mixtures thereof. With theproper choice of solvent, adhesion of the abrasion resistant layer canbe improved while minimizing migration of plasticizers and other addendafrom the low birefringence protective polymer film, enabling thehardness of the abrasion resistant layer to be maintained. Suitablesolvents for TAC low birefringence protective polymer film are aromatichydrocarbon and ester solvents such as toluene and propyl acetate.

The UV polymerizable monomers and oligomers are coated and dried, andsubsequently exposed to UV radiation to form an optically clearcross-linked abrasion resistant layer. The preferred UV cure dosage isbetween 50 and 1000 mJ/cm².

The thickness of the abrasion resistant layer is generally about 0.5 to50 micrometers preferably 1 to 20 micrometers, more preferably 2 to 10micrometers.

The abrasion resistant layer is preferably colorless, but it isspecifically contemplated that this layer can have some color for thepurposes of color correction, or for special effects, so long as it doesnot detrimentally affect the formation or viewing of the display throughthe overcoat. Thus, there can be incorporated into the polymer dyes thatwill impart color. In addition, additives can be incorporated into thepolymer that will give to the layer desired properties. Other additionalcompounds may be added to the coating composition, includingsurfactants, emulsifiers, coating aids, lubricants, matte particles,rheology modifiers, crosslinking agents, antifoggants, inorganic fillerssuch as conductive and nonconductive metal oxide particles, pigments,magnetic particles, biocide, and the like.

The abrasion resistant layer of the invention typically provides a layerhaving a pencil hardness (using the Standard Test Method for Hardness byPencil Test ASTM D3363) of at least 2H and preferably 2H to 8H.

The cover sheets of the invention may contain an antiglare layer, a lowreflection layer or an antireflection layer on the same side of thecarrier substrate as the low birefringence protective polymer film. Theantiglare layer, low reflection layer or antireflection layer is locatedon the opposite side of the low birefringence protective polymer film tothe layer promoting adhesion to PVA. Such layers are employed in an LCDin order to improve the viewing characteristics of the display,particularly when it is viewed in bright ambient light. The refractiveindex of an abrasion resistant, hard coat is about 1.50, while the indexof the surrounding air is 1.00. This difference in refractive indexproduces a reflection from the surface of about 4%.

An antiglare coating provides a roughened or textured surface that isused to reduce specular reflection. All of the unwanted reflected lightis still present, but it is scattered rather than specularly reflected.For the purpose of the present invention, the antiglare coatingpreferably comprises a radiation cured composition that has a texturedor roughened surface obtained by the addition of organic or inorganic(matting) particles or by embossing the surface. The radiation curedcompositions described hereinabove for the abrasion resistant layer arealso effectively employed in the antiglare layer. Surface roughness ispreferably obtained by the addition of matting particles to theradiation cured composition. Suitable particles include inorganiccompounds having an oxide, nitride, sulfide or halide of a metal, metaloxides being particularly preferred. As the metal atom, Na, K, Mg, Ca,Ba, Al, Zn, Fe, Cu, Ti, Sn, In, W, Y, Sb, Mn, Ga, V, Nb, Ta, Ag, Si, B,Bi, Mo, Ce, Cd, Be, Pb and Ni are suitable, and Mg, Ca, B and Si aremore preferable. An inorganic compound containing two types of metal mayalso be used. A particularly preferable inorganic compound is silicondioxide, namely silica.

Additional particles suitable for use in the antiglare layer of thepresent invention include the layered clays described incommonly-assigned U.S. patent application Ser. No. 10/690,123, filedOct. 21, 2003. The most suitable layered particles include materials inthe shape of plates with high aspect ratio, which is the ratio of a longdirection to a short direction in an asymmetric particle. Preferredlayered particles are natural clays, especially natural smectite claysuch as montmorillonite, nontronite, beidellite, volkonskoite,hectorite, saponite, sauconite, sobockite, stevensite, svinfordite,halloysite, magadiite, kenyaite and vermiculite as well as layereddouble hydroxides or hydrotalcites. Most preferred clay materialsinclude natural montmorillonite, hectorite and hydrotalcites, because ofcommercial availability of these materials.

The layered materials suitable for the antiglare layer may comprisephyllosilicates, for example, montmorillonite, particularly sodiummontmorillonite, magnesium montmorillonite, and/or calciummontmorillonite, nontronite, beidellite, volkonskoite, hectorite,saponite, sauconite, sobockite, stevensite, svinfordite, vermiculite,magadiite, kenyaite, talc, mica, kaolinite, and mixtures thereof. Otheruseful layered materials may include illite, mixed layeredillite/smectite minerals, such as ledikite and admixtures of illiteswith the layered materials named above. Other useful layered materials,particularly useful with anionic matrix polymers, may include thelayered double hydroxide clays or hydrotalcites, such asMg₆Al_(3.4)(OH)_(18.8)(CO₃)_(1.7)H₂O, which have positively chargedlayers and exchangeable anions in the interlayer spaces. Preferredlayered materials are swellable so that other agents, usually organicions or molecules, may splay, that is, intercalate and/or exfoliate, thelayered material resulting in a desirable dispersion of the inorganicphase. These swellable layered materials include phyllosilicates of the2:1 type, as defined in the literature (for example, “An introduction toclay colloid chemistry,” by H. van Olphen, John Wiley & SonsPublishers). Typical phyllosilicates with ion exchange capacity of 50 to300 milliequivalents per 100 grams are preferred. Generally, it isdesirable to treat the selected clay material to separate theagglomerates of platelet particles to small crystals, also calledtactoids, prior to introducing the platelet particles to the antiglarecoating. Predispersing or separating the platelet particles alsoimproves the binder/platelet interface. Any treatment that achieves theabove goals may be used. Examples of useful treatments includeintercalation with water-soluble or water insoluble polymers, organicreagents or monomers, silane compounds, metals or organometallics,organic cations to effect cation exchange, and their combinations.

Additional particles for use in the antiglare layer include polymermatte particles or beads which are well known in the art. The polymerparticles may be solid or porous, preferably crosslinked polymerparticles. Porous polymer particles for use in an antiglare layer aredescribed in commonly-assigned U.S. patent application Ser. No.10/715,706, filed Nov. 18, 2003.

In a preferred embodiment, particles for use in the antiglare layer havean average particle size ranging from 2 to 20 micrometers, preferablyfrom 2 to 15 micrometers and most preferably from 4 to 10 micrometers.They are present in the layer in an amount of at least 2 wt percent andless than 50 percent, typically from about 2 to 40 wt. percent,preferably from 2 to 20 percent and most preferably from 2 to 10percent.

The thickness of the antiglare layer is generally about 0.5 to 50micrometers preferably 1 to 20 micrometers more preferably 2 to 10micrometers.

Preferably, the antiglare layer has a 60° Gloss value, according to ASTMD523, of less than 100, preferably less than 90 and a transmission hazevalue, according to ASTM D-1003 and JIS K-7105 methods, of less than50%, preferably less than 30%.

In another embodiment of the present invention, a low reflection layeror antireflection layer is used in combination with an abrasionresistant hard coat layer or antiglare layer. The low reflection orantireflection coating is applied on top of the abrasion resistant orantiglare layer. Typically, a low reflection layer provides an averagespecular reflectance (as measured by a spectrophotometer and averagedover the wavelength range of 450 to 650 nm) of less than 2%.Antireflection layers provide average specular reflectance values ofless than 1%.

Suitable low reflection layers for use in the present invention comprisefluorine-containing homopolymers or copolymers having a refractive indexof less than 1.48, preferably with a refractive index between about 1.35and 1.40. Suitable fluorine-containing homopolymers and copolymersinclude: fluoro-olefins (for example, fluoroethylene, vinylidenefluoride, tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene,perfluoro-2,2-dimethyl-1,3-dioxol), partially or completely fluorinatedalkyl ester derivatives of (meth)acrylic acid, and completely orpartially fluorinated vinyl ethers, and the like. The effectiveness ofthe layer may be improved by the incorporation of submicron-sizedinorganic particles or polymer particles that induce interstitial airvoids within the coating. This technique is further described in U.S.Pat. No. 6,210,858 and U.S. Pat. No. 5,919,555. Further improvement ofthe effectiveness of the low reflection layer may be realized with therestriction of air voids to the internal particle space ofsubmicron-sized polymer particles with reduced coating haze penalty, asdescribed in commonly-assigned U.S. patent application Ser. No.10/715,655, filed Nov. 18, 2003.

The thickness of the low reflection layer is 0.01 to 1 micrometer andpreferably 0.05 to 0.2 micrometer.

An antireflection layer may comprise a monolayer or a multi-layer.Antireflection layers comprising a monolayer typically providereflectance values less than 1% at only a single wavelength (within thebroader range of 450 to 650 nm). A commonly employed monolayerantireflection coating that is suitable for use in the present inventioncomprises a layer of a metal fluoride such as magnesium fluoride (MgF₂).The layer may be applied by well-known vacuum deposition technique or bya sol-gel technique. Typically, such a layer has an optical thickness(i.e., the product of refractive index of the layer times layerthickness) of approximately one quarter-wavelength at the wavelengthwhere a reflectance minimum is desired.

Although a monolayer can effectively reduce the reflection of lightwithin a very narrow wavelength range, more often a multi-layercomprising several (typically, metal oxide based) transparent layerssuperimposed on one another is used to reduce reflection over a widewavelength region (i.e., broadband reflection control). For such astructure, half wavelength layers are alternated with quarter wavelengthlayers to improve performance. The multi-layer antireflection coatingmay comprise two, three, four, or even more layers. Formation of thismulti-layer typically requires a complicated process comprising a numberof vapor deposition procedures or sol-gel coatings, which correspond tothe number of layers, each layer having a predetermined refractive indexand thickness. Precise control of the thickness of each layer isrequired for these interference layers. The design of suitablemulti-layer antireflection coatings for use in the present invention iswell known in the patent art and technical literature, as well as beingdescribed in various textbooks, for example, in H. A. Macleod, “ThinFilm Optical Filters,” Adam Hilger, Ltd., Bristol 1985 and James D.Rancourt, “Optical Thin Films User's Handbook,” Macmillan PublishingCompany, 1987.

The cover sheets of the invention may also contain a moisture barrierlayer. The moisture barrier layer comprises a hydrophobic polymer suchas a vinylidene chloride polymer, vinylidene fluoride polymer,polyurethane, polyolefin, fluorinated polyolefin, polycarbonate, andothers, having a low moisture permeability. Preferably, the hydrophobicpolymer comprises vinylidene chloride. More preferably, the hydrophobicpolymer comprises 70 to 99 weight percent of vinylidene chloride. Themoisture barrier layer may be applied by application of an organicsolvent-based or aqueous coating formulation. To provide effectivemoisture barrier properties the layer should be at least 1 micrometer inthickness, preferably from 1 to 10 micrometers in thickness, and mostpreferably from 2 to 8 micrometers in thickness. The cover sheet of theinvention comprising a moisture barrier layer has a moisture vaportransmission rate (MVTR) according to ASTM F-1249 that is less than 1000g/m²/day, preferably less than 800 g/m²/day and most preferably lessthan 500 g/m²/day. The use of such a barrier layer in the cover sheet ofthe invention provides improved resistance to changes in humidity andincreased durability of the polarizer plate comprising the cover sheet,especially for TAC cover sheets having a thickness less than about 40micrometers.

The cover sheets of the invention may contain a transparent antistaticlayer. The antistatic layer aids in the control of static charging thatmay occur during the manufacture and use of the cover sheet composite.Effective control of static charging reduces the propensity for theattraction of dirt and dust to the cover sheet composite. The guardedcover sheet composite of the invention may be particularly prone totriboelectric charging during the peeling of the cover sheet from thecarrier substrate. The so-called “separation charge” that results fromthe separation of the cover sheet and the substrate can be effectivelycontrolled by an antistatic layer having a resistivity of less thanabout 1×10¹¹ Ω/square, preferably less than 1×10¹⁰ Ω/square, and mostpreferably less than 1×10⁹ Ω/square.

Various polymeric binders and conductive materials may be employed inthe antistatic layer. Polymeric binders useful in the antistatic layerinclude any of the polymers commonly used in the coating art, forexample, interpolymers of ethylenically unsaturated monomers, cellulosederivatives, polyurethanes, polyesters, hydrophilic colloids such asgelatin, poly(vinyl alcohol), polyvinyl pyrrolidone, and others.

Conductive materials employed in the antistatic layer may be eitherionically-conductive or electronically-conductive. Ionically-conductivematerials include simple inorganic salts, alkali metal salts ofsurfactants, polymeric electrolytes containing alkali metal salts, andcolloidal metal oxide sols (stabilized by metal salts). Of these,ionically-conductive polymers such as anionic alkali metal salts ofstyrene sulfonic acid copolymers and cationic quaternary ammoniumpolymers of U.S. Pat. No. 4,070,189 and ionically-conductive colloidalmetal oxide sols which include silica, tin oxide, titania, antimonyoxide, zirconium oxide, alumina-coated silica, alumina, boehmite, andsmectite clays are preferred.

The antistatic layer employed in the current invention preferablycontains an electronically-conductive material due to their humidity andtemperature independent conductivity. Suitable materials include:

(1) electronically-conductive metal-containing particles includingdonor-doped metal oxides, metal oxides containing oxygen deficiencies,and conductive nitrides, carbides, and bromides. Specific examples ofparticularly useful particles include conductive SnO₂, In₂O, ZnSb₂O₆,InSbO₄, TiB₂, ZrB₂, NbB₂, TaB₂, CrB, MoB, WB, LaB₆, ZrN, TiN, WC, HfC,HfN, and ZrC. Examples of the patents describing these electricallyconductive particles include; U.S. Pat. Nos. 4,275,103; 4,394,441;4,416,963; 4,418,141; 4,431,764; 4,495,276; 4,571,361; 4,999,276;5,122,445; and 5,368,995;

(2) fibrous electronic conductive particles comprising, for example,antimony-doped tin oxide coated onto non-conductive potassium titanatewhiskers as described in U.S. Pat. Nos. 4,845,369 and 5,166,666,antimony-doped tin oxide fibers or whiskers as described in U.S. Pat.Nos. 5,719,016 and 5,0731,119, and the silver-doped vanadium pentoxidefibers described in U.S. Pat. No. 4,203,769; and

(3) electronically-conductive polyacetylenes, polythiophenes, andpolypyrroles, preferably the polyethylene dioxythiophene described inU.S. Pat. No. 5,370,981 and commercially available from Bayer Corp. asBAYTRON POLYETHYLENE DIOXYTHIOPHENE.

The amount of the conductive agent used in the antistatic layer can varywidely depending on the conductive agent employed. For example, usefulamounts range from about 0.5 mg/m² to about 1000 mg/m², preferably fromabout 1 mg/m² to about 500 mg/m². The antistatic layer has a thicknessof from 0.05 to 5 micrometers, preferably from 0.1 to 0.5 micrometers toinsure high transparency.

The cover sheets of the invention may contain a viewing anglecompensation layer (also referred to as a compensation layer, retarderlayer, or phase difference layer), with proper optical properties,between the PVA dichroic film and liquid crystal cell, such as disclosedin U.S. Pat. Nos. 5,583,679; 5,853,801; 5,619,352; 5,978,055; and6,160,597. A compensation film according to U.S. Pat. Nos. 5,583,679 and5,853,801 based on discotic liquid crystals which have negativebirefringence, is widely used.

Compensation films are used to improve the viewing angle characteristic,which describes a change in contrast ratio from different viewingangles. It is desirable to be able to see the same image from a widevariation in viewing angles and this ability has been a shortcoming withliquid crystal display devices. The primary factor limiting the contrastof a liquid crystal display is the propensity for light to “leak”through liquid crystal elements or cells, which are in the dark or“black” pixel state. Furthermore, the leakage and hence contrast of aliquid crystal display are also dependent on the direction from whichthe display screen is viewed. Typically the optimum contrast is observedonly within a narrow viewing angle range centered about the normalincidence to the display and falls off rapidly as the viewing directiondeviates from the display normal. In color displays, the leakage problemnot only degrades the contrast but also causes color or hue shifts withan associated degradation of color reproduction.

Viewing angle compensation layers useful in the present invention areoptically anisotropic layers. The optically anisotropic, viewing anglecompensation layers may comprise positively birefringent materials ornegatively birefringent materials. The compensation layer may beoptically uniaxial or optically biaxial. The compensation layer may haveits optic axis tilted in the plane perpendicular to the layer. The tiltof the optic axis may be constant in the layer thickness direction orthe tilt of the optic axis may vary in the layer thickness direction.

Optically anisotropic, viewing angle compensation layers useful in thepresent invention may comprise the negatively birefringent, discoticliquid crystals described in U.S. Pat. Nos. 5,583,679, and 5,853,801;the positively birefringent nematic liquid crystals described in U.S.Pat. No. 6,160,597; the negatively birefringent amorphous polymersdescribed in commonly assigned U.S. Patent Application Publication2004/0021814A and U.S. patent application Ser. No. 10/745,109, filedDec. 23, 2003. These latter two patent applications describecompensation layers comprising polymers that contain non-visiblechromophore groups such as vinyl, carbonyl, amide, imide, ester,carbonate, sulfone, azo, and aromatic groups (i.e. benzene, naphthalate,biphenyl, bisphenol A) in the polymer backbone and that preferably havea glass transition temperature of greater than 180 degree C. Suchpolymers are particularly useful in the compensation layer of thepresent invention. Such polymers include polyesters, polycarbonates,polyimides, polyetherimides, and polythiophenes. Of these, particularlypreferred polymers for use in the present invention include: (1) apoly(4,4′-hexafluoroisopropylidene-bisphenol)terephthalate-co-isophthalate; (2) apoly(4,4′-hexahydro-4,7-methanoindan-5-ylidene bisphenol) terephthalate;(3) a poly(4,4′-isopropylidene-2,2′6,6′-tetrachlorobisphenol)terephthalate-co-isophthalate; (4) apoly(4,4′-hexafluoroisopropylidene)-bisphenol-co-(2-norbornylidene)-bisphenolterephthalate; (5) apoly(4,4′-hexahydro-4,7-methanoindan-5-ylidene)-bisphenol-co-(4,4′-isopropylidene-2,2′,6,6′-tetrabromo)-bisphenolterephthalate; (6) apoly(4,4′-isopropylidene-bisphenol-co-4,4′-(2-norbornylidene)bisphenol)terephthalate-co-isophthalate;(7) apoly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene)bisphenol)terephthalate-co-isophthalate;or (8) copolymers of any two or more of the foregoing. A compensationlayer comprising these polymers typically has an out-of-planeretardation, R_(th), that is more negative than −20 nm, preferablyR_(th) is from 60 to −600 nm, and most preferably R_(th) is from −150 to−500 nm.

Another compensation layer suitable for the present invention includesan optically anisotropic layer comprising an exfoliated inorganic claymaterial in a polymeric binder as described in Japanese PatentApplication 11095208A.

The auxiliary layers of the invention can be applied by any of a numberof well known liquid coating techniques, such as dip coating, rodcoating, blade coating, air knife coating, gravure coating, microgravurecoating, reverse roll coating, slot coating, extrusion coating, slidecoating, curtain coating, or by vacuum deposition techniques. In thecase of liquid coating, the wet layer is generally dried by simpleevaporation, which may be accelerated by known techniques such asconvection heating. The auxiliary layer may be applied simultaneouslywith other layers such as subbing layers and the low birefringenceprotective polymer film. Several different auxiliary layers may becoated simultaneously using slide coating, for example, an antistaticlayer may be coated simultaneously with a moisture barrier layer or amoisture barrier layer may be coated simultaneously with a viewing anglecompensation layer. Known coating and drying methods are described infurther detail in Research Disclosure 308119, Published December 1989,pages 1007 to 1008.

The cover sheets of the invention are suitable for use with a widevariety of LCD display modes, for example, Twisted Nematic (TN), SuperTwisted Nematic (STN), Optically Compensated Bend (OCB), In PlaneSwitching (IPS), or Vertically Aligned (VA) liquid crystal displays.These various liquid crystal display technologies have been reviewed inU.S. Pat. Nos. 5,619,352 (Koch et al.), 5,410,422 (Bos), and 4,701,028(Clerc et al.).

FIG. 10 presents a cross-sectional illustration showing one embodimentof a typical liquid crystal cell 260 having polarizer plates 252 and 254disposed on either side. Polarizer plate 254 is on the side of the LCDcell closest to the viewer. Each polarizer plate employs two coversheets. For the purpose of illustration, polarizer plate 254 is shownwith an uppermost cover sheet (this is the cover sheet closest to theviewer) comprising a layer promoting adhesion to PVA 261, tie layer 262,low birefringence protective polymer film 264, barrier layer 266, andantiglare layer 268. The lowermost cover sheet contained in polarizerplate 254 comprises a layer promoting adhesion to PVA 261, tie layer262, low birefringence protective polymer film 264, barrier layer 266,and viewing angle compensation layer 272. On the opposite side of theLCD cell, polarizer plate 252 is shown with an uppermost cover sheet,which for the purpose of illustration, comprises a layer promotingadhesion to PVA 261, tie layer 262, low birefringence protective polymerfilm 264, barrier layer 266, and viewing angle compensation layer 272.Polarizer plate 252 also has a lowermost cover sheet comprising a layerpromoting adhesion to PVA 261, tie layer 262, low birefringenceprotective polymer film 264, and barrier layer 266.

Another aspect of the present invention is directed to a method offorming a polarizing plate comprising:

(A) providing two guarded cover sheet composites each comprising:

-   -   (i) a carrier substrate; and    -   (ii) a protective cover sheet as in claim 1

(B) providing a poly(vinyl alcohol)-containing dichroic film; and

(C) simultaneously or sequentially bringing each protective cover sheetinto contact with said poly(vinyl alcohol)-containing dichroic film suchthat the layer promoting adhesion to a poly(vinyl alcohol)-containingfilm in each protective cover sheet is in contact with said poly(vinylalcohol)-containing dichroic film.

The invention is also directed to a method not involving a carrier sheetin which the method comprises forming a polarizing plate comprisingproviding two cover sheets as described above, providing a poly(vinylalcohol)-containing dichroic film, and simultaneously or sequentiallybringing said cover sheets into contact with said poly(vinylalcohol)-containing dichroic film such that the layer promoting adhesionto a poly(vinyl alcohol)-containing film in each of said two coversheets is in contact with said poly(vinyl alcohol)-containing dichroicfilm. The present invention is illustrated in more detail by thefollowing non-limiting examples.

EXAMPLES Synthesis of UV-Absorbing Polymers

UV Absorbing Polymer Latex 1:

A stirred reactor containing 125 g of water and 4.3 g of Rhodaplex®CO-436 surfactant (58% solids) was heated to 85° C. and purged with N₂for 0.5 hour. Then, 1.25 g of potassium persulfate and 0.25 g of sodiummetabisulfate were added to the reactor. An emulsion containing 600 g ofdeionized water, 188 g of NORBLOC 7966 (Ciba) UV monomer, 50 g of ethylacrylate, 12.5 g of methacrylic acid, 4.3 g of RHODAPLEX CO-436 (58%solids) surfactant, 1.25 g of potassium persulfate and 0.25 g of sodiumwas added continuously for 1 hour. The reaction was allowed to continuefor 4 more hours before the reactor was cooled down to room temperature.A UV absorbing polymer latex was filtered through glass fiber to removeany coagulum. The resultant UV absorbing polymer latex had a solid ofabout 25% and the particle had a mean size of about 80 nm and acomposition of 75% by weight of Norbloc® 7966, 20% by weight of ethylacrylate, and 5% by weight of methacrylic acid.

UV Absorbing Polymer Latex 2:

A stirred reactor containing 125 g of water and 4.3 g of Rhodaplex®CO-436 surfactant (58% solids) was heated to 85° C. and purged with N₂for 0.5 hour. Then, 1.25 g of potassium persulfate and 0.25 g of sodiummetabisulfate were added to the reactor. An emulsion containing 600 g ofdeionized water, 188 g of NOBLOC 7966 (Ciba), 50 g of ethyl acrylate,12.5 g of methacrylamide, 4.3 g of RHODAPLEX CO-436 (58% solids), 1.25 gof potassium persulfate and 0.25 g of sodium was added continuously for1 hour. The reaction was allowed to continue for 4 more hours before thereactor was cooled down to room temperature. The UV absorbing polymerlatex was filtered through glass fiber to remove any coagulum. Theresultant UV-absorbing polymer latex had a solid of about 25% and theparticle had a mean size of about 80 nm and a composition of 75% byweigh of NORBLOC 7966, 20% by weight of ethyl acrylate, and 5% by weightof methacrylamide.

Example 1 Invention

A 100 micrometer thick poly(ethylene terephthalate) (PET) carriersubstrate having an antistatic backing layer (backside) is coated on itsfront surface with an adhesion-promoting layer comprising CERVOL 205 PVA(polyvinyl alcohol having a degree of hydrolysis of about 88-89%,available from Celanese Corp.) having a dry coating weight of about 100mg/ft² (1000 mg/m²), and UV absorbing latex polymer 1, as describedabove, having a dry coating weight of about 83 mg/ft² (830 mg/m²). Thedried layer is then overcoated with a triacetyl cellulose (TAC)formulation comprising three layers: a surface layer comprisingCA-438-80S (triacetyl cellulose from Eastman Chemical) having a drycoating weight of about 208 mg/ft² (2080 mg/m²), diethyl phthalatehaving a dry coating weight of about 20.8 mg/ft² (208 mg/m²), andSURFLON S-8405-S50 (a fluorinated surfactant from Semi Chemical Co. Ltd)having a dry coating weight of about 21 mg/ft² (210 mg/m²); a mid layercomprising CA-438-80S having a dry coating weight of about 1899 mg/ft²(18990 mg/m²), SURFLON S-8405-S50 having a dry coating weight of about29.5 mg/ft² (295 mg/m²), diethyl phthalate having a dry coating weightof about 284 mg/ft² (2840 mg/m²) and TINUVIN 326 having a dry coatingweight of about 83 mg/ft² (830 mg/m²); a lower layer as the tie layercomprising a mixture of 95:5 cellulose acetate trimellitate(Sigma-Aldrich) and trimethyl borate and having a dry coating weight ofabout 100 mg/ft² (1000 mg/m²). The TAC formulation was applied with amulti-slot slide hopper using a mixture of methylene chloride andmethanol as the coating solvent.

The dried TAC coating was peeled off from the PET carrier substrate atthe interface between the front side of the carrier substrate and thelayer promoting adhesion to PVA film. The peeling was very smooth andthe peeled TAC film had very low haze and a good appearance that wasfree from wrinkles.

Example 2 Invention

The Example was prepared in a similar manner as the Invention Example 1except that the adhesion-promoting layer comprised CERVOL 205 PVA(polyvinyl alcohol having a degree of hydrolysis of about 88-89%,available from Celanese Corp.) having a dry coating weight of about 100mg/ft² (1000 mg/m²), and UV absorbing latex polymer 2, as describedabove, having a dry coating weight of about 83 mg/ft² (830 mg/m²).

The dried TAC coating was peeled off from the PET carrier substrate atthe interface between the front side of the carrier substrate and thelayer promoting adhesion to PVA film. The peeling was very smooth andthe peeled TAC film had very low haze and a good appearance that wasfree from wrinkles.

Example 3 Comparison

The Comparative Example 2 was prepared in a similar manner as theInvention Example 1 except that the adhesion-promoting layer comprisedonly CERVOL 205 PVA at a dry coating weight of about 100 mg/ft² (1000mg/m²), and the mid layer comprising CA-438-80S having a dry coatingweight of about 1899 mg/ft² (18990 mg/m²), SURFLON S-8405-S50 having adry coating weight of about 29.5 mg/ft² (295 mg/m²), diethyl phthalatehaving a dry coating weight of about 284 mg/ft² (2840 mg/m²), TINUVIN8515 UV absorber having a dry coating weight of about 65 mg/ft² (650mg/m²), and PARSOL 1789 UV absorber having a dry coating weight of about6.5 mg/ft² (65 mg/m²)

The dried TAC coating was peeled off from the PET carrier substrate atthe interface between the front side of the carrier substrate and thelayer promoting adhesion to PVA film. The peeling was very smooth andthe peeled TAC film had a good appearance that was free from wrinklesbut an unacceptable level of haze.

The above examples clearly demonstrate that the present invention hasovercome the limitations of prior art polarizer cover sheets andeliminated the need for complex surface treatments such assaponification prior to the fabrication of polarizer plates.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST

-   10 coating and drying system-   12 moving carrier substrate/web-   14 dryer-   16 coating apparatus-   18 unwinding station-   20 back-up roller-   22 coated substrate-   24 guarded cover sheet composite-   26 wind-up station-   28 coating supply vessel-   30 coating supply vessel-   32 coating supply vessel-   34 coating supply vessel-   36 pump-   38 pump-   40 pump-   42 pump-   44 conduit-   46 conduit-   48 conduit-   50 conduit-   52 discharge device-   54 polar charge assist device-   56 nip roller-   58 nip roller-   60 preformed strippable protection layer-   62 unwinding station-   64 wind up station-   66 drying section-   68 drying section-   70 drying section-   72 drying section-   74 drying section-   76 drying section-   78 drying section-   80 drying section-   82 drying section-   92 front section-   94 second section-   96 third section-   98 fourth section-   100 back plate-   102 inlet-   104 1^(st) metering slot-   106 pump-   108 lowermost layer-   110 inlet-   112 2^(nd) metering slot-   114 pump-   116 layer-   118 inlet-   120 metering slot-   122 pump-   124 layer-   126 inlet-   128 metering slot-   130 pump-   132 layer-   134 inclined slide surface-   136 coating lip-   138 2^(nd) inclined slide surface-   140 3^(rd) inclined slide surface-   142 4^(th) inclined slide surface-   144 back land surface-   146 coating bead-   151 guarded cover sheet composite-   153 guarded cover sheet composite-   159 guarded cover sheet composite-   162 lowermost layer-   164 intermediate layer-   166 intermediate layer-   168 uppermost layer-   170 carrier substrate-   171 coversheet-   173 cover sheet-   174 lowermost layer-   176 intermediate layer-   178 intermediate layer-   179 cover sheet-   180 uppermost layer-   182 carrier substrate-   184 release layer-   186 lowermost layer-   187 intermediate layer-   188 intermediate layer-   189 cover sheet-   190 uppermost layer-   200 feed line-   202 extrusion hopper-   204 pressurized tank-   206 pump-   208 metal drum-   210 first drying section-   212 drying oven-   214 cast film-   216 final drying section-   218 final dried film-   220 wind-up station-   232 guarded cover sheet composite supply roll-   234 guarded cover sheet composite supply roll-   236 PVA dichroic film supply roll-   240 carrier substrate take-up roll-   242 pinch roller-   244 pinch roller-   250 polarizer plate-   252 polarizer plate-   254 polarizer plate-   260 LCD cell-   261 layer promoting adhesion to PVA-   262 tie layer-   264 low birefringence protective polymer film-   266 barrier layer-   268 antiglare layer-   272 viewing angle compensation layer

What is claimed is:
 1. A method of forming a polarizing platecomprising: (A) providing at least one guarded cover sheet compositescomprising: (i) a carrier substrate; and (ii) a protective cover sheetcomprising a low birefringence protective polymer film having in-planebirefringence, Δn_(in) of less than 1×10⁻⁴ and an out-of-planebirefringence Δn_(th) of 0.005 to −0.005 and, in addition, one or morefunctional layers, which layers are either adjacent or non-adjacent eachother, wherein the one or more functional layers comprise at least anadhesion-promoting layer for adhering a poly(vinyl alcohol)-containingfilm to the low birefringence protective polymer film, wherein thefunctional layers do not comprise a polarizing film, wherein theadhesion-promoting layer which is a functional layer comprises aUV-absorbing polymer, and wherein the adhesion-promoting layer is coatedonto the carrier substrate and then dried, prior to application of thelow birefringence protective polymer film; (B) providing a poly(vinylalcohol)-containing dichroic film; (C) removing the carrier substratefrom the guarded cover sheet composite after formation of the protectivecover sheet; and (D) bringing the protective cover sheet into contactwith the poly(vinyl alcohol)-containing dichroic film such that theadhesion-promoting layer in the protective cover sheet is in contactwith the poly(vinyl alcohol)-containing dichroic film.
 2. The methodaccording to claim 1, wherein said carrier substrate is a polymericfilm.
 3. A method of forming a polarizing plate comprising: providingtwo guarded cover sheet composites each comprising a protective coversheet comprising a low birefringence protective polymer film havingin-plane birefringence, Δn_(in) of less than 1×10⁻⁴ and an out-of-planebirefringence Δn_(th) of 0.005 to −0.005 and, in addition, one or morefunctional layers, which layers are either adjacent or non-adjacent eachother, wherein the one or more functional layers comprise at least anadhesion-promoting layer for adhering a poly(vinyl alcohol)-containingfilm to the low birefringence protective polymer film, wherein thefunctional layers do not comprise a polarizing film, wherein theadhesion-promoting layer which is a functional layer comprises aUV-absorbing polymer, and wherein the adhesion-promoting layer is coatedonto a carrier substrate and then dried, prior to application of the lowbirefringence protective polymer film; providing a poly(vinylalcohol)-containing dichroic film, and simultaneously or sequentiallybringing said cover sheets into contact with said poly(vinylalcohol)-containing dichroic film such that the adhesion-promoting layerfor adhering a poly(vinyl alcohol)-containing film to the lowbirefringence protective polymer in each of the two cover sheets is incontact with the poly(vinyl alcohol)-containing dichroic film, andremoving carrier substrates from the guarded cover sheet compositesafter formation of the protective cover sheets.
 4. The method accordingto claim 3, wherein said carrier substrates are polymeric films.